It is no revelation that prison inmates strut on stage as to the manner born. Performance brings out the inner thespian in everyone, and conjures dreams in offenders whose lives have hitherto been a shambles. Charitable companies (Pimlico Opera, Bruce Wall's London Shakespeare Workout) that give prisoners their chance to shine this way always impress audiences. But afterwards, as the jangling keys release them from the Scrubs or Wandsworth or Holloway, they may ask, what happens next?

Today, one such charity, Only Connect, demonstrates its long-term commitment to former prisoners by opening its own theatre in London. It is housed - largely thanks to a grant from the Mercers' Company - in a handsome converted chapel near King's Cross. The first production is John Steinbeck's The Grapes of Wrath, including some specially commissioned songs and with a cast of 20 who were all, until very recently, guests of Her Majesty. Steinbeck's great saga about the Joad family, including son Tom out on parole, journeying from the Oklahoma dustbowl to the promise of California in the 1930s, offers ideal roles for actors familiar with dispossession and destructive lives.

Only Connect is run by Danny Kruger and his wife of six months, Emma Ashcroft. Kruger is a policy adviser to David Cameron, and it was he who drafted Cameron's “hug-a-hoody” speech. Kruger and Ashcroft, a drama teacher, are Alpha-course Christians whose evangelical zeal informs their commitment to ex-offenders: they maintain contact with all the actors - 40 so far - in their excellent previous productions, Of Mice and Men at Wormwood Scrubs, and Steel Magnolias at Holloway.

The biggest challenge with ex-prisoners is fostering reliability. Which means getting them to turn up to rehearsals. As Ashcroft says: “Simply being strict with them just doesn't work. Everyone has to be worked with individually, and with understanding of the various problems they all have. You can get commitment and teamwork only from positive relationships.”

So they have taken their commitment farther. Ashcroft and Kruger live for part of each week with some of their actors in a rented house near by. Ashcroft says: “They trust us, and that's how we build relationships. They respond to our passion. We form a bond with them, and give them the feeling of belonging to a community.”

Their house-mother is Katie Pedder, a 31-year-old Cambridge graduate who was previously the Governor of Wormwood Scrubs and finds her new role almost as demanding. “I don't want to be policing you,” she tells them, “so stick to the rules.” These include no smoking, no drugs, no visitors staying over, helping with chores and respecting each other. And there is prayer: “The word prayer comes naturally,” Ashcroft says. “They have Jamaican grandmothers who pray, so they're pleased that I, white and middle-class, pray too.”

Last week, Indira Varma, the young actress currently starring in David Hare's The Vertical Hour at the Royal Court, spent an afternoon at the Only Connect theatre, supervising a rehearsal. She threw her considerable energy into giving the actors professional tips about projection, clarity, communication, using their imagination. She had to be tough (“Take your hands out of your pockets!” three times) but she realised that they needed constant coaxing and cajoling. All actors are told to “leave their issues at the door” when performing, but ex-prisoners don't find this simple. She told them: “You've got this amazing space, here - and a great story to perform.”

I watched these ex-offenders, between 24 and 41, fit athletic men meekly taking direction, some of them trying to overcome feelings of inadequacy, others all too eager to get up there and ham it up. Talking to them later, finding them intelligent, articulate, positive, grateful, I was relieved not to have met them on a dark night when they were still pursuing their criminal lives - addicted to crack cocaine, burgling, fighting, supplying class-A drugs. Time will tell whether they are as clean or as straight as they now profess.

One, Vince Martin, who plays Pa Joad, had given the judge at Harrow Crown Court, Barrington Black, a heartfelt promise that if his rehab in Weston-super-Mare helped him to get off drugs, he would fulfil the judge's trust and justify the expense. That was 11 months ago. For the moment at least, he and all the others have all been given a fantastic chance to prove themselves.

Playing Tom Joad's sister Roseof-Sharon is Kimberley Gooch, who is only 22 and was released from Holloway in August after serving ten months for robbery, her first offence. While in Holloway she was in the cast of Steel Magnolias - and got a standing ovation. She is now qualified in garden design and maintenance, and all her family will be there to watch her tonight.

The Grapes of Wrath opens today at the Only Connect Theatre, 32 Cubitt Street, London WC1, at 2.30pm, with further performances tonight and every night this week, Mon-Fri, at 7.30pm. Tickets £12.50 and £9 (0844 4771000)

Editor's choice

• The End: the worst movie endings ever
• Introducing The Times Proms Podcast
• Heath Ledger tipped for Oscar