It is a cold, crisp Friday morning, and the school hall is packed with almost 200 sixth-formers. There is none of the usual chatting at the back, no fidgeting and no gazing distractedly around the room. All eyes are fixed on the figure at the front of the room.

John Pridmore, 40, former East End gangster and hard man is a far from conventional choice for a visiting speaker for schools. But the audience is rapt as he tells the pupils: “At 13 I began stealing, at 15 I was in a detention centre, at 19 I was in prison.” By the time he was 27, he was involved in drug dealing and protection rackets all over London and habitually carried a machete and a can of CS spray. As a nightclub bouncer, he earned a small fortune from shady deals, which he spent on drink, drugs, women and fast cars. But he is here today in an attempt to steer the pupils away from such a path. A religious conversion 13 years ago changed Mr Pridmore’s life; he became a Catholic, gave away his dubiously acquired gains and decided to devote his time to working with young people across Britain.

The venue for today’s talk is St James’ Catholic high school in the north London borough of Barnet, one of the stops on a 30-day speaking tour of schools, parish halls, community centres and prisons. Mr Pridmore cuts an imposing figure. More than 6ft tall and built like the bouncer he once was, he is dressed in a dark blue suit, with a shaved head and a small goatee beard. He could easily still pass as a gangster and, as we shake hands, I fear my fingers will be crushed in his palm.

But appearances can be deceptive; he is disarmingly gentle, charming and witty. His devotion to his faith stands out. “I’m not here to convert people,” he says. “I’m about sharing my story.” And although he maintains that his own journey to rock bottom and back is solely thanks to finding God, even the most secular individuals can relate to the issues he raises.

“Many of them will probably be sitting there thinking how glamorous the gangster life is. But it isn’t; it’s sick,” he says. “I want to destroy the myths.”

Few would deny that this is a valid message, but surely there might be concerns about inviting a convicted criminal into schools to deliver it? “We had a few worries initially, because we didn’t know how it would work,” says St James’s head of religious education, Tom McDonald, who organised the talk. “But outside speakers can bring real freshness to a subject, and if they are good at speaking to young people, it can be very effective,” he says. “Young people can see through phonies, but they love someone who is genuine.”

Mr Pridmore starts with a brief summary of his life from the age of 11, when he discovered his parents were getting divorced. “I came home from school to be told I had to decide who to live with,” he says. “I decided not to love any more.” He continues, through the beatings from his stepmother, detention centre, jail and drugs. “I’m not telling you this to glamorise my past,” he insists. “There isn’t a lot of glory in being told to cut someone up.”

He recounts brief tales of gang violence, but doesn’t labour the point. The audience soon gets the picture of the sort of activities he was involved in (recounted in gruesome detail in his book From Gangland to Promised Land) and, as Mr Pridmore says, he wants to protect at least an element of the students’ innocence. But if the extreme violence seems oceans away from this school on a Friday morning, other issues are certainly closer to home.

“I was smoking dope like it was going out of fashion,” he tells the audience. “Everyone says it’s great chilling out with a joint, but what’s great about paranoia? What’s great about staring at the wall for hours like a vegetable?”

There is some uncomfortable shuffling around on chairs. “I took ecstasy, some of which I knew could be laced with rat poison. Then I moved on to crack cocaine. Everyone thinks they can handle the drugs, but if you take drugs, at some point in your life you will realise they have destroyed you.” He tells the teenagers about a friend: “He has been in a mental hospital for 22 years. He took so many drugs it scrambled his brain. He has been in an institution since he was 18 years old.”

Then he moves on to sex. “You will meet guys who tell you to sleep with them, and who will try to use you,” he tells the girls. “Don’t fall for it.

If he does love you, he will respect you, and will wait for you. Boys,” he continues, “don’t treat women that way. It is destructive to you, and to them.” Coming from someone whose book suggests that in the bad old days he used to change his women as often as his socks, this is quite an about-turn. As the talk ends, many of the pupils rush to hug Mr Pridmore, and ask the TES photographer if they can have their picture taken with him.

Christina Fitzgerald, 17, says it is Mr Pridmore’s life experiences that made the talk so powerful. “He doesn’t deny that he’s done bad things, but he turned his life around. That is much more effective than someone who hasn’t been there coming in and saying, ‘don’t do this’ or ‘don’t do that’.”

The girls become a little more coy when asked what they thought of his advice about sex, but they are generally approving. “What he said about women and sex: I felt he really understood; it was very relevant to us,” says Christina. The boys agree. “He definitely hit the button in terms of talking to young people,” says Gennaro Alvino, 17.

But do Mr Pridmore’s talks work? “I have been to some of the roughest schools, in places like Liverpool and Belfast, and we have had reports of young people making changes in their lives after the talk, moving away from drugs, and that sort of thing,” he says.

Such direct change may happen, but the effects of his story on most young people are difficult to quantify. “We will see over time what it has done for the students,” says Mr McDonald. “But he has certainly created a stir.”

John Pridmore’s autobiography, From Gangland to Promised Land is published by Xt3, pound;7.95. For more information, tel Xt3: 01937 579730