It is often complained that British writers no longer engage with urgent social issues. Novelists have retreated into the past, or have become so self-consciously literary that they are almost unreadable. The theatre is increasingly trivial or archaic. Yet right under our noses all along, there's been one great writer who has left barely a social issue untouched. The death of socialism, the Troubles, paedophilia, rape, the decline of religion, poverty, heroin - you name it, he has written about it, and for an audience of millions on our most popular medium of all: television.

Jimmy McGovern's latest project, the Channel 4 film Sunday, which marks the 30th anniversary of the Bloody Sunday massacre, has already generated huge controversy. He has been attacked, for example, for leaving Martin McGuinness out of the film. For his research, he met McGuinness. "But that was before all these accusations that he fired the first shot," he explains, referring to the claim made by an unnamed source to Lord Saville's inquiry into the massacre. McGuinness was "open", McGovern says, "but he never told me anything new. He's shrewd. He didn't tell me anything he hadn't said to anyone else."

He is unrepentant about excluding McGuinness from his narrative. "It's not a film about the IRA, it's a film about the people who took part in an anti-internment march and were shot dead by the British army. This confusion is wrong, I feel." As for McGuinness, "I don't think he was in the Bogside that day. I think he was halfway between the Bogside and Craigie, because the thought at the time was that all the IRA men of the area were on the march, so the Craigie was wide open to the British army." So McGuinness, he believes, was occupied elsewhere - but because he couldn't prove this, he didn't put it in the film.

The critics who have implied, predictably, that McGovern is being blindly pro-nationalist couldn't be more wrong. Unprompted, he shows empathy for the situation in which the British soldiers found themselves that day. "I think even now [the British soldiers] would say that they were doing their duty . . . I think they thought they were doing exactly what was expected of them. They were frightened and they were convinced there was going to be a whole load of snipers there. They thought they were going in and there would be the enemy standing in the street. They were used to Belfast. They didn't know Derry . . . They were tired and they were sick of Ireland. They had been in Ireland for 20 months, and they were expecting to go home."

Later on, he adds that, in his twenties, he "knew one lad who was a wonderful footballer, and he joined the paratroopers, and he was sent over there [to Northern Ireland]. And he was murdered by the IRA." McGovern was acutely aware, while working on the film, that he was "an Englishman writing something that would reflect badly on my country". These are hardly the words of a man blinded by his green sympathies.

Yet McGovern is clear that, however understandable their initial panic, the soldiers unforgivably lied in the aftermath. "They lied. They all lied. They were called to identify how they'd shot, where they'd shot, and told to draw lines [along the trajectory of the bullets]. In the end, they were all absolutely pissing themselves laughing, drawing lines in this great cobweb." He spoke to a paratrooper who was there on the day, who explained that the mood afterwards among the soldiers was one of "total elation". "They thought they'd done a good job. They thought they did their duty and did it well. It's incredible."

However, the responsibility for the massacre doesn't lie only with soldiers on the ground. It runs to the very top of the British government. "There was a meeting on the Friday prior to Bloody Sunday between [the Unionist prime minister Brian] Faulkner and [the then British prime minister Edward] Heath, and what was said at that meeting, nobody knows. It's interesting that happened only hours before [the killings]. Even if Heath was blissfully unaware [that is, he didn't order the killings], though, he did say to Widgery [whom he appointed to conduct the first, discredited investigation into Bloody Sunday], 'we're fighting a propaganda war over there'. Now, Heath will explain that, I'm sure, but it's obvious what Lord Widgery thought he meant: give me a whitewash."

McGovern sees Heath's decisions - including his order of internment against the advice of the army - as a crucial turning point in the Troubles. It's so easy to see the vicious sectarian conflict as inevitable, almost carried in the blood, but he points out that, "in 1969, [James] Callaghan went down the Bogside and addressed the crowds from a bedside window". It didn't have to turn into a bloodbath; political decisions made it so.

As a poor kid growing up in Liverpool, McGovern had gained insights into the divisions between Protestants and Catholics. Nothing, however, prepared him for what he found when he began research for Sunday. One man in particular sticks in his mind. "He was fierce, out for revenge, any form of revenge. You know - poison the water. He'd lost friends that day. He was frightening. But there was a lot like that. He was the kind of person I felt I had to interview."

McGovern was initially mystified as to why Lord Saville allowed the film to go ahead while he was conducting his investigation. "I'm sure he had the power to stop it if he wanted to. I kept thinking: what's in it for Saville, except a pain in the arse?" But Lord Saville's inquiry - which is supposed to be independent and conducted on an international basis - has twice been overruled by a British court. "So I think Saville probably allowed us to go ahead to affect the mood over here, so that next time it's not so easy for a British court to overrule him."

There are obvious parallels between Bloody Sunday and the subject of one of McGovern's earlier films, about the Hillsborough stadium tragedy. Both events forced the families of the victims to defend their dead. Both featured, he argues, "a cover-up, and a judge who was a disgrace and created a denial of justice". But with Hillsborough, "the worst charge is manslaughter. I'm an extremist on Hillsborough and even I wouldn't charge the South Yorkshire Police with murder. But with Bloody Sunday, there's overwhelming evidence that it was murder."

Hillsborough was a turning point for McGovern's politics. It was the moment at which he realised that the hard left "didn't care about us or our class. They hated us . . . I firmly believe that if you say to people on the left, 'white working-class male', they picture a football supporter with a spanner in his hand, about to riot. But if you say to me, 'working-class male', I think cultured trade unionists - not fine culture, but, you know, books and music and internationalism. The best example is the dockers [the subject of McGovern's 1999 film Dockers]. They were brilliant over Nelson Mandela. Anything shipped from South Africa, they refused to handle. That's all forgotten now. White working-class males aren't a sexy cause to champion."

McGovern has been one of the few writers putting on TV the stories of British white working-class men. He does this in part because he believes they are a neglected group. "In the Eighties, I found it difficult to be a white working-class male. You were always perceived as racist, homophobic, sexist . . . I used to know good trade unionists, good socialists, who were perceived as racists just because they were football fans." It was this neglect of working-class men, by both the right and the left, that "led to Hillsborough. You could have police treating them like utter shit . . . they thought we were scum, so they could get away with any lies."

McGovern's political views are informed by a wider philosophical analysis. "All the problems of our country," he explains, "stem from the fact that we outline a principle like justice, say, or faith, and we build a big institution around it, and then we protect the institution at the expense of the principle. Now Widgery is a perfect example . . . It is the belief of the British people in justice that enabled Widgery to sit in that room with Ted Heath and Lord Hailsham [then Lord Chancellor]. But what did he do? He shits on justice. But why? He was a man at the very top of the tree. You couldn't bribe him, you couldn't offer him anything. But he still shat on justice because that's what he thought the British Prime Minister expected of him."

There is one mouth-watering slice of McGovern's rage that, sadly, we will never see. He wrote a script about the other tragedy that defines working-class Liverpool in the public consciousness: the murder of James Bulger. After six months, he "threw it in the bin".

His fictionalised version had become too similar to the real events for him to feel comfortable putting it on TV. He felt it would be too hurtful for the Bulgers and the people of Liverpool. His view remains controversial in the city: he believes that Jon Venables and Robert Thompson should never have been tried in an adult court.

He argues that "to achieve real justice that will make those lads come to terms with the enormity of their crime, they have to grow and have a child of their own and raise that child with love and affection. Only then will they see what they've done. That's a terrible sentence."

He is - as always - painfully aware of the hurt he might cause by saying this. He says that, "if I was the Bulgers, I'd attack me for saying it".

Jimmy McGovern is a folk hero in Liverpool. I wandered around the shops and cafes for half an hour asking Liverpudlians about him, and they all spoke as though he were a personal friend. When, later, we wandered into public places, more than one person who recognised him seemed able to start chatting warmly to him.

It's about time this local hero went national.

Sunday will be screened on Channel 4, at 9pm, on 28 January.

For an extended version of this interview visit, www.newstatesman.co.uk