Prisoner rehabilitation is not the most fashionable of causes for pop stars to espouse. Which is why you have to admire singer Billy Bragg's efforts to recruit his colleagues on to a project offering hope to those locked up in Britain's jails.

Bragg has persuaded the likes of the Foo Fighters guitarist Chris Shiflett, the Clash co-founder Mick Jones and Dirty Pretty Things to stage a number of low-key concerts for inmates, a world away from the backslapping of most charity events. These have now been captured in Breaking Rocks, a new documentary about the charity Jail Guitar Doors, set up by Bragg to supply prisoners with guitars and the skills to use them, in order to help them on the outside.

In the film/ Jones and Bragg perform a version of the Clash's "Should I Stay Or Should I Go" and Bob Marley's "Redemption Song" at Brixton and Wormwood Scrubs Prisons in London. After one visit Jones says: "The guys were telling us how much this scheme had helped them move on from their previous lives before prison. It was really touching to think we've helped, even if it's in a small way."

The initiative is attracting a growing number of performers. Shiflett has led a guitar class at Brixton, while the indie band Hard-Fi hope to perform inside Feltham Young Offender Institution in west London -- the setting of "Feltham Is Singing Out", their song about the suicide of a petty thief on remand. Dirty Pretty Things held a workshop inside Brixton, and the Lincoln band Eastroad played at HMP Stocken, in Rutland.

But the principal aim of Bragg's charity is not to stage star-studded concerts, but to raise money for instruments. Guitars have already been donated to HMP Styal -- Britain's largest women's prison -- as well as Guys Marsh in Dorset, Pentonville, Wandsworth, the Verne on the Isle of Portland, Wormwood Scrubs and Reading Young Offender Institution.

Bragg believes that mastering a guitar can offer a way out of the reoffending cycle into which so many prisoners fall. "I support punishment," he says, "but I also believe in rehabilitation. Prison has to be about much more than just locking people up. We want people to move on from their situation and reconnect with the outside world. Learning to play and write gives them self-confidence, which is very important in cutting reoffending. We're preparing them to deal with what life throws at them in a non-confrontational way."

Bragg and the film's director, Alan Miles, plan to screen the documentary -- whose title is taken from the opening words of the Bobby Fuller Four's classic "outlaw" song "I Fought the Law" -- at a number of jails next year. In one of the film's most moving scenes, Bragg introduces a former prisoner on to the stage at the Glastonbury Festival. The man, recently paroled from nearby HMP Shepton Mallet, soon has the crowd cheering as he performs a song he composed behind bars.

"The lifers at Shepton Mallet Prison over the hill hear the festival at night," Bragg tells them. "There are guys there who play guitar, and as musicians they know, as I know, that a guitar will help you transcend your surroundings and find a release."

"Breaking Rocks" is being screened at the Shortwave Cinema in Bermondsey, south-east London, on Sunday 22 November, then at venues nationwide in February.

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