As important as his new poetic form are the political passions which invigorate it. D'Aguiar sees Johnson in a radical tradition embracing Swift, Shelley and Clare; a return to the poet on the front line. He was the first to give voice to the "second generation" of black Britons - children of Windrush-era postwar West Indian migrants. According to the writer Caryl Phillips, he was the "first crossover voice, who made it possible for a generation to think of themselves as black and creative in literature, music, the media". For many, he has been an alternative poet laureate, chronicling the black experience in Britain over three decades, while reflecting on events from South Africa to eastern Europe.

A Spectator profile in 1982 found that such poetry as Johnson's, with its use of Jamaican patois and phonetic spelling, had "wreaked havoc in schools and helped to create a generation of rioters and illiterates". But by the 1990s he was on the cover of Critical Quarterly, praised in Poetry Review and translated into German and Italian. He spawned an international school of dub poetry, while inspiring British poets, from Benjamin Zephaniah and Lemn Sissay to Simon Armitage. For Christina Patterson, director of the Poetry Society, he has "had a radical impact on British poetry in the past 30 years, changing the ways poetry is perceived; many young poets wouldn't be around without him".

This month Johnson joins Frost and Graves with publication of his selected poems, Mi Revalueshanary Fren, as a Penguin Modern Classic, only the second living poet (after the nonagenarian Czeslaw Milosz) to appear in this hallowed list. His editor, Ellah Allfrey, believes there has been time enough to assess his lasting value. "His is a uniquely British form of language," she says. "This is the English language coming back home, changed." The poet Carol Ann Duffy welcomes Penguin's move as "an inspired and inspirational publishing moment". But while the Daily Telegraph noted with surprise on its front page "Reggae radical joins Betjeman", the TLS diarist griped that some readers "may find the ushering of Linton Kwesi Johnson into the circles of the immortals a little premature". "People are edgy because Penguin is messing with the canon," says Phillips, professor of English at Columbia University in New York.

Michael Schmidt, editorial director of Carcanet Books, though an admirer of Johnson's recitals, deems his work a classic not of "literary" but "performance" poetry, best captured on CD or video. "It doesn't belong on the page," he insists. "It's patronising - misappropriating something that thrives in a different medium." For others, the distinction between page and stage is artificial and ghettoising: rhyme and metre are about the voice, whether spoken or heard in the reader's mind. As Lemn Sissay has said, "If you are a black poet, the literary establishment automatically decides that you must be a performance poet."

Johnson, who once wrote an ironic poem entitled, "If I Waz a Tap Natch Poet", can be his own sternest critic. "If it was down to me, the book would be thinner: there's stuff I wrote when I was young. I'd have been happy to have been published by Penguin, full stop." He is pleased his work will for the first time be distributed by a major publisher and is enjoying a surge of attention (a BBC4 profile was broadcast on Thursday). Yet he seems resigned to carping. "Someone on Radio 4 said it's cynical commercialism because I'm marketable. It's exactly the kind of reaction I was anticipating; I'm only surprised it's not worse."

Johnson has a reputation for seriousness, yet also reveals wit and warmth. In a tribute poem, "Seasons", Jean Binta Breeze likened him to a "cole front/cross de Atlantic" until he smiles, "an yuh haffi sey/it did wut it/after all/fi endure im winta". For him poetry has always gone hand in hand with political activism. He has lived, worked and campaigned for almost 40 years in Brixton, south London, where he is known simply as "Poet". He shares a house-cum-office in Railton Road with his partner and PA, Sharmilla Beezmohun, in the now-gentrified "front line" of the 70s and early 80s. One of Britain's first black journalists and broadcasters - most recently he presented BBC Radio 3's Caribbean Evening last October - he is also a music producer with his own company, LKJ records, and his own manager. In a "quiet" year he tours for three to five months, and is planning trips to Brazil and Martinique as well as an imminent national tour.

He retains respect not least because he has declined to water down his political views for commercial gain. "I've always been an independent-minded person and I wanted to have control over my creativity, to do things on my own terms," he says. "I've made a success of that." Phillips says, "You know he's not owned by anyone, and that's a remarkable position for a creative black person to have established."

Linton Johnson was born in 1952 in Chapelton, a small town in the British colony of Jamaica. His grandparents were peasant farmers the land could no longer support. His mother Sylvena was a domestic, "washing clothes for single men"; his father Eric, a baker and sugar-estate worker, was one of her customers. The couple separated when Linton was seven (he has five half-sisters and two half-brothers). Sylvena emigrated to Britain shortly before Jamaican independence in 1962 and Linton spent three years with his maternal grandmother, "dirt poor but happy, farming, tending animals, harvesting sugar cane", before joining his mother in Brixton, aged 11. When his father died in 1982, he wrote the moving elegy "Reggae fi Dada", blaming social conditions for his death.

Johnson found London cold and ugly. "It wasn't the picture-book idea one has of the mother country." Yet he was at ease among Brixton's large Jamaican community, and "happy because I was with my mother". A machinist and part-time dressmaker, Sylvena remarried and, since retiring in 1988, has lived in Jamaica's Montego Bay. "She was a hard-working woman throughout her life; I worship the ground she walks on," Johnson says. "If there's any good things about me, I got them from her."

He was a top pupil in Jamaica, but, like most black children of his generation, relegated to the bottom stream in his Tulse Hill secondary school, a period he recalls as "traumatic: kids and teachers were racist; you'd get into fights all the time". Although one teacher urged him to follow the Jamaican prime minister Michael Manley to the London School of Economics "in general, they had low expectations and didn't take kindly if they thought you harboured ambitions above your station". Aware that "education was the only way out of poverty for somebody like myself", he sought mentors elsewhere.

As black consciousness spread in the US and Caribbean he joined the British Black Panthers. "Although our slogan was 'black power, people's power', we weren't anti-white. The ideas were working-class solidarity - giving people power over their lives. As a teenager I became passionate about freedom, equality, justice." In the Panthers' library he discovered WEB Du Bois's The Souls Of Black Folk, Franz Fanon's The Wretched Of The Earth, and Eric Williams's Capitalism And Slavery. "I found books written by black people, about black people - those were exciting times."

Johnson left school at 17 with six O-levels to get married to Barbara, the daughter of his mother's hairdresser. "My wife was pregnant with our first child but I was determined to continue my education," he says. He went to night school while doing clerical work, including a spell as a civil servant in the Treasury. As a sociology student in the mid-70s at Goldsmiths College, London, he temped ("when I turned up they'd say there was some mistake"), then worked on building sites. Unemployed for six months after graduating ("I was 'overqualified' or not qualified enough"), he worked on an assembly line to support his family. These experiences fuelled his early poetry, including the collection "Inglan Is A Bitch" (1980). He says, "Those were very, very hard times."

Johnson ran a Panthers' poetry workshop while at school, and worked with the drummers Rasta Love. A Brixton priest sent him to John La Rose, the Trinidadian poet and lawyer who co-founded the Caribbean Artists' Movement in 1966 and ran the New Beacon Bookshop in north London. La Rose was impressed by "Linton's maturity and responsibility - he was married and working at 18". He introduced him to seminal influences: poets of the negritude movement, Aimé Césaire and David Diop; Felix Tchikaya U'Tamsi, Martin Carter, Edward Kamau Brathwaite, Christopher Okigbo, and African-Americans such as Langston Hughes, Gwendolyn Brooks and Amiri Baraka.

"I wanted to do with Jamaican speech what Americans like the Last Poets were doing," says Johnson. But he used reggae in place of jazz and social commentary, akin to the way to Jamaican DJs, such as I Roy, U Roy and Big Youth did. There were blues dances every Saturday night at unlicensed, all-night reggae clubs with booming sound systems, which were often raided by police. "Jamaican music was the source of an independent identity," he says. "Reggae wasn't just about love songs but society and spiritual nourishment." He took the name Kwesi, meaning "born on Sunday", but preferred "sowshallism" and pan-Africanism to Rastafarianism. When he later met Bob Marley in London, Marley asked "why I was so militant and why I wasn't a Rasta".

Ferdinand Dennis, author of "Behind The Frontlines" and "The Last Blues Dance", sees Johnson as a bridge between Caribbean and black British culture. "He borrowed Rastafarian forms to express alienation, pressure, the lack of direction of the first British-born generation in the '70s and the attempt to contain them by the forces of law and order. Here was somebody who had his finger on the pulse; you sensed he was experiencing the same pressures his poetry expressed."

Johnson recalls Brixton street life as an oasis of resistance and rebellion. His generation was "criminalised by the 'sus' laws" - the precursor to today's stop and search - and there was "endless harassment". He was arrested in 1972. "I saw three youths being manhandled by police in Brixton market. As a Black Panther I was trained to take the policemen's numbers. I was thrown into a black maria and given a good kicking, then charged with GBH and assault. When they saw a big 'free Linton' demonstration outside the police station, they increased the charges." He was acquitted and thanks, he says, to an Observer article by the South African writer Lewis Nkosi, headlined "Why blacks in Brixton are blowing their tops", the officers involved were transferred.

His poetry collections, Voices Of The Living And The Dead (1974), Dread, Beat An Blood (1975) and Inglan Is A Bitch (1980) marked the harsh coming of age of a generation for whom they became anthems. For Caryl Phillips, "There was nobody else articulating what was going on in the streets of Britain for young black people." The poem "Five Nights of Bleeding", lamenting the knife fights between promoters of rival sound systems as "rebellion rushing doun di wrong road", drew on Fanon's view of the internalising of oppression: "Fanon wrote about violence in the anti-colonial struggle. I transferred that to what was happening around me, the violence at the hands of the police and among black youth. Thirty years on it hasn't changed; now they're killing each other with guns."

His poem Sonny's Lettah movingly protested against the sus laws, through a son's message to his mother from Brixton prison, while Forces of Victri celebrated the Notting Hill carnival, the scene of clashes with police in the mid-70s. "Fite Dem Back" was a response to the burgeoning National Front in the era of Enoch Powell's 1968 "rivers of blood" speech, and Margaret Thatcher's pandering, in the run-up to her election in 1979, to fears of being "swamped by people of a different culture". For the poet and novelist David Dabydeen, professor of Caribbean studies at Warwick university, Johnson "captured a deeply disturbing sense of dread and violence, with images of broken glass, blood, bleak oppression". D'Aguiar has recalled the mesmerising quality of "vintage LKJ... the serious expression, the gravelly voice and deep monotone delivery, the chant and mantra style".

Inspired partly by the African-American Paul Lawrence Dunbar - the first collection of poems he ever saw in dialect - Johnson combined dread talk and Rasta speech with inner-city patois. He had precursors in Jamaican poets such as Louise Bennett and Claude McKay, and later acknowledged a debt to Kamau Brathwaite, the Bajan advocate of "nation language" who declared, "the hurricane does not howl in iambic pentameter". Yet for Dabydeen, Johnson's was a "unique marriage of Caribbean Creole and black urban patois; a defiant, unapologetic assertion of black language as a mode for poetic expression".

Dennis says, "at school, you were made to feel ashamed of your language. But when you heard Linton Johnson you became aware of its power, that it was something to be proud you owned, as well as standard English." As Johnson says, "There's no such thing as bad English: there's English and ways of speaking it. A lot of early poets would have been writing in local dialects; was Chaucer standard English, or Robbie Burns?" For Phillips, Johnson "opened the door for a generation of writers to feel free to use patois, nation language, whether from an ethnic group or regional background".

In 1977 Johnson won a Cecil Day Lewis fellowship as writer in residence in the London borough of Lambeth, and was education officer at the Keskidee Arts Centre in Islington, an early home of black theatre. As a music reviewer he wrote record sleeve blurbs for the fledgling Virgin Records, which he persuaded to record his first album in 1977, Poet And The Roots. He used backing tracks, then the live Dennis Bovell Dub Band. Bovell, the musical director and multi-instrumentalist whom Johnson calls the "Quincy Jones of reggae music", arranged Johnson's compositions. Music was a "vehicle for bringing in a wider, young audience". With the Virgin album Dread Beat An Blood (1978), and Island Records' Forces Of Victory (1979) and Bass Culture (1980), his fame surged amid reggae's rising popularity. Riding on a punk-reggae "anti-establishment" coalition, he opened for punk groups, such as Johnny Rotten's PIL - successor to the Sex Pistols ("I came on to a frightening sea of punks shouting for Johnny, but I stood my ground") - and Siouxsie and the Banshees.

"He burst on the scene like a meteor," says Dabydeen. Seen as a spokesman for the grievances of a generation, he was the subject of a BBC Omnibus documentary, Dread Beat An Blood, but it was barred from broadcast until after the 1979 election, since one poem attacked "Maggi Tatcha on di go/wid a racist show/but a she haffi go". "Di Great Insohreckshan" celebrated the 1981 uprisings in Bristol, Liverpool and London, where in Brixton the police operation "Swamp '81" proved the final straw. "It was euphoric: there was a sense that, at last, we were fighting back," he says. "People couldn't take it any more." Some accused the poet of inciting violence. Others saw him as having been prescient. In Johnson's view "you didn't have to be a prophet to know what happens if you keep pouring oil on a fire: the beatings, brutalisation, frame-ups".

He wrote campaigning poems: for George Lindo, framed for robbery in Bradford and later released with £25,000 compensation; Blair Peach, the New Zealand teacher killed by police at an anti-racist rally in 1979; New Craas Massakah, about the fire that killed 14 young party-goers in 1981, when "di whole a black Britn did rack wid rage". He was a steward on the Black People's Day of Action in 1981, when 20,000 people marched in protest at the handling of the fire investigation, a march he sees as a watershed which "made the British establishment sit up and see we were a force to be reckoned with - that things had to change". According to La Rose, he has a "reputation beyond the politics he espoused, a broader popularity based on his visionary approach".

Johnson founded his own company, LKJ records, in 1981, to produce "thinking man's reggae, instead of a pulse from a computer". He turned down Island Records' Chris Blackwell: "He offered me a six-album record deal, and I said no, which was unheard of. They wanted to spend $1 million to make me into a big star, but I didn't want to be in debt to a record company." He gave the company one more record, Mekin Histri (1984), which, though his other early albums still sell, is no longer available. Johnson deems it his most overtly political album. "Maybe that's why they suppressed it." Though limited distribution is "the price you pay for keeping control", he has no regrets. "I'm my own man," he smiles.

"Being signed to a record company would take the edge off the poetry," says Bovell, "because you're vying with Top of the Pops - that's how they're going to market you." According to Phillips, "any major record company would have signed him in the late '70s and '80s. But Linton didn't invest anything in being a pop star. He's schooled in radical politics, in not being co-opted." Johnson promoted other dub poets, including Jean Binta Breeze and the late Michael Smith, and musicians. For Phillips, "Linton moves without any sense of ego, not feeling his work would be compromised by showing support for anyone else's."

Johnson was arts editor of Race Today, and in 1982 presented a pioneering 10-part series on Jamaican music, From Mento To Lovers Rock, on BBC Radio 1. In the mid-80s he reported for Channel 4's Bandung File, covering subjects from the Labour MP Bernie Grant's election campaign to the Angolan war. But "the money wasn't anything like the music business". He quit to go touring again in 1988: "We've been on the road ever since."

"He's great company when he's relaxed, but he's very often guarded," says the Liverpool poet Brian Patten, who has toured with him. "Poetry is more a means to an end for Linton. He's a deeply serious man." But the collection Tings And Times (1991) was more humorous and personal, including ironic reflections on the collapse of the Soviet bloc. It hinted at "ow rebels get ole ow/some sell dem soul", while "Di Anfinish Revalueshan" insisted, "we noh reach mount zion/yet". Johnson regrets that a third generation "takes things for granted without realising people had to fight for them".

His BBC2 Counterblast programme in 1999, Independant Intavenshan, took a historical view to warn against complacency. "Between the Scarman report [following the 1981 riots] and the Macpherson report [on the Stephen Lawrence inquiry], we saw the emergence of a black middle class," he says. "That first batch of MPs came to power on the back of the [1981] black uprisings, whether they want to admit it or not." Often scathing about "di black petti boorshwa" that "side with the opressah when the going get ruf", he says: "Some people believe we've won and they've arrived. I'm not one of them; we have other struggles."

Johnson and his wife, a secretary who later did a sociology degree at Goldsmiths, divorced in 1994. They have one daughter, Karen, a nursery teacher (and mother of Johnson's grandchild, Darnell), and two sons: Eric, an aspiring writer; and Marcos, a Seventh Day Adventist who is studying theology. Johnson, who renounced religion in his teens, says of his younger son: "A lot of his friends are in prison or selling drugs; I'm a lot happier with what he's doing."

He met Sharmilla Beezmohun, who was then in publishing, at a Groucho Club charity event in 1995. "She's been my right and my left hand. Since she began working for me my life has been less stressful... She's been good for me." He professes simple pleasures: reading, cooking, listening to music - often jazz. "I fool around with my bass [guitar], go to my local for a few pints, play dominoes." On TV he favours "soaps and current affairs" but has no time for Ali G ("If a black person was doing what he's doing, he'd be accused of being anti-semitic"). He reveals a hedonistic side in his relish of good food and wine ("one of the pleasures of touring"). According to Patten, "oysters are Linton's drug of choice".

Though he describes the break-up of his marriage as "very amicable", it gave rise to more private poems, such as Hurricane Blues, about a "period of emotional trauma, separation and divorce". Worried that he may have limited himself, he says, "I'm known for public poetry. If I write love poems, people will think it's out of character." Phillips disagrees: "Even in the earlier poems there's always a deeply personal subtext. He's just bringing to the surface something that was always there, his passion for people." Johnson says, "Lately I've found myself writing mostly elegies. Perhaps it's getting older." "Reggae fi Bernard" was for a nephew who worked for British Rail and was killed in a mysterious accident. "Reggae fi May Ayim" was for an Afro-German friend who committed suicide, while his most recent was for the late Bernie Grant - "genuinely a man of the people".

But one poem, "Story", describes unease "wen yu cyan fin de rime fi fit di beat". "I'm not writing as much as I used to," he admits. "The initial motivation came out of the struggle. As things have changed, the urgency has diminished... Story is about the frustration of wanting to write and not having the space. Poetry is demanding; the muse is like a jealous lover who wants attention; I can't give it - I'm running a record company or on tour." While there are moments of inspiration, when "poems happen like magic, as though you're a vessel", he says "it gets harder because I've become more conscious about craft. Initially it was wanting to get something off my chest when I was angry or deeply moved. But you have to work at it."

According to Dabydeen, Johnson spawned "hundreds of inferior followers who thought mouthing off in front of a microphone was enough". Johnson has been scornful of a performance poetry: "Bandwagon: it was hip to promote young black poets who were spouting rhetoric. But you have to set standards. I've come across people who call themselves poets but they don't read. I can't imagine someone calling themselves a musician without listening to music." Some feel Johnson is stuck in a reggae groove while other British poets draw more on hip-hop. Yet Johnson has warned against apeing the US, dismissing black nationalists such as Louis Farrakhan as representing "reverse racism", and crude Afrocentrics: "Black kings and queens? Who wants to identify with absolute rulers?"

He plans to release a live album this year, and volume three of LKJ In Dub. He has toyed with taking time off from touring. "But I say, make hay while the sun shines: today, you're Linton Kwesi Johnson; tomorrow," he screws up his eyes, "you're 'Clinton who?'." His 1998 album was entitled More Time, and Johnson faces his 50th birthday this August with ambivalence. "I feel good within myself to know I've lived this long. When you're involved in revolutionary struggle, you're wondering if you're going to die in a police cell. I never thought I'd live to the age I have now, but you become more aware of your own mortality." While being designated a "classic" is a quiet source of satisfaction, Johnson says: "I don't think I've written my best verse yet. I hope I have a few poems left in me."

Life at a glance: Linton Kwesi Johnson

Born: August 24 1952; Chapelton, Jamaica.

Education: Tulse Hill secondary school, south London; 1973-76 Goldsmiths College (BA sociology).

Married: 1970-94 Barbara (one daughter, Karen; two sons, Eric and Marcos).

Career: 1977 writer in residence, Lambeth; '78 Education officer, Islington;'81- founding head, LKJ Records and Music Publishers; arts editor RaceToday; '85-88 reporter Bandung File; '91- trustee, George Padmore Institute.

Poetry books: 1974 Voices Of The Living And The Dead; '75 Dread, Beat An Blood; '80 Inglan Is A Bitch; '91 Tings an Times; 2002 Mi Revalueshanary Fren.

Some albums: 1977 Poet And The Roots; '78 Dread Beat An Blood; '80 Bass Culture; '84 Mekkin Histri; '91 Tings An Times; '92 LKJ in Dub, vol II; '96 A Cappella Live; '98 More Time.

· Mi Revalueshanary Fren was published this week by Penguin Modern Classics at £6.99.

· Linton Kwesi Johnson tours the UK from May 9 to July 10. For details tel: 01684 540 366