The authorial voice behind Criminal Justice, a compelling new BBC drama, is a barrister-turned-writer. Natural affinities between the two professions abound - there are laws in scriptwriting just as there is scriptwriting in the law. But the path that links the two is not often trodden, and when it is, the footprints of John Mortimer tend to stamp out everyone else's.

Mortimer's great creation Rumpole of the Bailey was an expression of his reverence for English justice, honed over centuries of enlightenment. Peter Moffat's dramas take the view that the legal system is of dodgier ethical fibre. Nabokov's Gloves, his debut play premiered at Hampstead Theatre ten years ago, was a jolting saga of conscienceless barristers who will sell themselves to the devil to secure a not-guilty verdict. His first drama for television was North Square, set in chambers run by scheming clerks. When it was unjustly discontinued after one series, Moffat was sufficiently discouraged to seek his tales of betrayal and moral dubiety elsewhere. He adapted Macbeth for the BBC's series of updated Shakespeare plays. In Cambridge Spies he mined the motives of Burgess, Maclean, Philby and Blunt.

Criminal Justice finds him confidently back on home turf. It dramatises the bewildering odyssey of a 21-year-old defendant (played by Ben Whishaw) as he passes through the criminal justice system from the moment he is accused of murder through to the verdict read out by the jury foreman. Without resorting to formula, its five episodes take a tour through the staples of genre drama - coppers, detectives, solicitors, barristers and prisoners - as they are brought into play.

“What's it like to leave a police station and be taken to the magistrate's court? What is a magistrate's court like? There are 50 of those little things that we have very little chance to look at in TV drama. I was just really anxious to get that texture, colour, detail over.”

To crank up the tension, his asthmatic protagonist is played by Ben Whishaw, whose look of blue-eyed hurt gives him the air of someone who will crumple on impact with the criminal justice system. “He has to be as normal as possible, in an environment he has never been in before, so that watching it we could feel, 'this could be me.'”

And so the system goes to work on him. Though convinced of his own innocence, the evidence is against him: the murder weapon found on his person when he is caught fleeing the scene of the crime. A detective tries to coax a confession out of him outside the interview room. A sleazy duty solicitor advises him to say nothing, so that his defence can be made up at a later date. Once remanded he enters a dehumanising prison system which makes no separate provision for detainees whose guilt is unestablished. And then there's the bizarre ordeal of being defended by lawyers who tell him what to say.

Moffat's desire to dramatise the workings of a system compromised by human frailty and economic imperative was initially stimulated by his own memories of defending clients of whom he knew nothing. “It is a very odd one. You meet your client only once, probably only for an hour before you represent them at a trial, and then your relationship is shaped by what you need them to be saying in order to fit what the defence is going to be.” As for his relationship with the police, it was restricted to cross-examination. “You're trying to make them look stupid or appear to be a liar. If you're defending as I was that's where your relation with police begins and ends.” Not that relations between police and prosecuting barristers are on a much firmer footing: policemen, he adds, refer to the crown's lawyers as the Can't Prosecute Service.

“It occurred to me I was spending all my time in this world without scratching below the surface,” he says. To supplement his knowledge of the courts, Moffat spent time in an Essex police station and also visited prison. “I didn't expect to be shocked by what I heard and saw. Both institutions are more extraordinary than I ever thought or believed.” In the police stations it wasn't so much the residual sexism and racism that surprised him as the casual acknowledgement that suspects can still be leant on. “I heard police officers say to me, and bear I mind that they know I'm going to write a drama for the BBC, 'Well there's always the time between cell and interview room, dot dot dot.'”

As for prison, his interviews with ex-cons and screws yielded a portrait of a system that begins its process of dehumanising the moment the accused is walled up in a prison van. It doesn't get any better when he is on the wing. “There are just very shocking levels of understaffing in prisons,” says Moffat. “I've heard prison officers say: 'There are only six officers on the wing tonight so they have to run things according to the easiest and best way of keeping order.'”

That means ceding control to prisoners who derive feudal powers from their control of the black market in drugs and mobile phones. Moffat swears by the truthfulness of one scene in which a returning convict is made to squat naked for 30 seconds and still manages to keep his anally smuggled mobile from popping out. “It seems to be a very common way of getting things in. Wardens can't come into contact with a person who is being body-searched or it might constitute an assault. One very high-status prisoner who ought to have felt relatively safe said to me that there was never a night when the cell door closed that he didn't feel a huge sense of relief he'd survived another day.”

Once the trial starts, Moffat's old professional skills and his new one melt into each other. “Because it's an adversarial system, because we have to put up a defence and say ‘this is what we're saying', you have to establish what the defence is, be clear about it, make every moment in court relate to that - in other words create a structured narrative that a jury can understand, buy into and believe. Inevitably what happens is that square pegs and round holes need to be made to fit. You're constructing this narrative which may or may not have a big relationship with the truth.”

It's up to viewers to decide what sort of relationship Criminal Justice has with the truth. But in his defence its creator would like to submit in evidence his years at the bar.

Criminal Justice, Mon-Fri, BBC One, 9pm

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