In an ideal world, Peter Kosminsky would not exist. “I do tend to be a moderately apocalyptic type of person,” he explains, in a voice so soft, you scarcely notice the steel. “My job is to point out the problems, not necessarily to see the solutions.” And there are always problems. The Tories were good to Kosminsky, allowing him to make his name with angry drama-documentaries spotlighting social and political ills that cropped up on their watch. The aggregate impression left by his films about the Stalker affair, prison suicide, bullying in schools and child prostitution was of a director determined to beat his audience over the head. It was only at the dawn of the Blair age that Kosminsky truly emerged as a basilisk-eyed observer of our moral health.

Warriors, an award-winning account of the traumatic fall-out of peacekeeping in Bosnia, served as a prequel to a trilogy of films in which Kosminsky has tracked the ethical degradation of the new Labour decade. In The Project, he dramatised the curdling of idealism occasioned by Millbank’s win-at-all-costs skulduggery. The Government Inspector anatomised Blair’s era-defining decision to go to war on the evidence of the dodgy dossier that cost David Kelly his life. And now comes Britz, a considered attempt, in the aftermath of July 2005, to fathom why young Muslims might seek to take up suicidal jihad against the country of their birth.

You can always tell when Kosminsky has turned in a new film from the fulminating of the Establishment. Britz will be no different. He has already asked himself whether it is appropriate to humanise suicide bombers. “Does it show any kind of consideration for the relatives of people who died on 7/7? I ended up thinking, well, yes – we don’t do them a service by portraying these people as insane. That would be as silly as saying the entire German nation was in the grip of a mass psychosis during the latter half of the 1930s. It’s clear there are second-generation Muslims who are contemplating extreme action who are not monsters. I think the way to do a service to the relatives and friends of those who died on 7/7 is to try to stop it happening again. And the first step is to try to understand how it could have happened in the first place.”

The two parts of Britz focus in turn on Sohail and Nasima, adult siblings born in Bradford and raised by traditional immigrants. The brother is assimilationist, expressing his gratitude to Britain by joining an MI5 hungry for Muslim recruits. His sister, however, is radicalised when a fellow medical student falls foul of the repressive new antiterror legislation. Kosminsky is a firm believer in exhaustive research, but it proved impossible to track down and interview British Muslims who answered to either description.

So, he turned for dramatic inspiration to an alternative source: his own divided loyalties as the son of an immigrant. “All my life, I have felt a battle within me,” he says. “On the one hand, the desire to be as British as I can be. At public school and Oxford, it was easy to hide my nonBritish antecedents. On the other hand, there is the desire to be proud of my antecedents and to overturn the applecart.” He also admits to having based Nasima partly on the imaginary younger sister he invented for himself as a child and later resurrected in bedtime stories for his two daughters. “She had the courage of her convictions in a way I often didn’t,” he unblushingly explains. He can’t have counted on a little sister strapping explosives into a fake pregnancy pouch.

This is a new side to Kosminsky. I first met him in 1999, when he was filming Warriors in the Czech Republic. The determination in his hawklike physiognomy grew even grimmer as, that very week, Nato started raining bombs on cast members’ relatives in Belgrade. Visiting him later, in his eyrie in Wiltshire, where he works all evening and all weekend, I was put in mind of St Augustine alone in his study. There is a joyless monasticism in his dogged conquest of stacks of research. But today’s Kosminsky, sunk into an armchair at the Groucho Club, is bonhomie itself. The bunker mentality has gone. It must have something to do with the fact that this is the first time Kosminsky has actually written his own script from scratch.

But it’s not just that. For the first time, Kosminsky has had no opposition as he went about making his film. “Refreshingly for me, since I’m used to being stonewalled by the authorities on my previous projects, MI5 were very helpful. They have a press office, and I think they have been told that it is their job to improve the public perception of the security service. I might have been the inadvertent beneficiary. I don’t want in any way to slag off colleagues’ work, but they’re pretty grumpy about Spooks.”

Of all the films Kosminsky has made about institutions and their questionable consciences, about the moment when public policy has an impact on private lives, he is in little doubt that Britz amounts to the most urgent statement of his career. “I don’t like it, I have to be honest, when our elected representatives are stupid. The way they are behaving seems absolutely guaranteed to generate the reaction that has been generated. If I were a politician, after global warming I would probably make it my No 1 priority to work slowly to undermine the sense of anger and disillusionment a million British Muslims currently feel, because I think we’re sitting on a real powder keg.”

Britz, C4, October 31 and November 1

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