It’s hardly going to break box office records, but Britain’s Forgotten Prisoners might just be the most important film made in the UK this year. For anybody interested in justice – or rather injustice – it’s unmissable.

Premiering at the Sheffield DocFest on Thursday, Britain’s Forgotten Prisoners tells the story of convicts serving indeterminate sentences, known as imprisonment for public protection (IPP) in England and Wales. It focuses on life after the prisoners have been released and their almost inevitable recall to prison, usually for minor breaches of their licence such as being late for an appointment with their probation officer.

IPP was introduced in 2003 by a Labour government determined to show it was tough on crime and tough on the causes of crime. Prisoners were given a tariff (a minimum term) but most have been jailed way beyond that, with no sense of when they would be freed. Indeed, it came to be regarded as so tough that it was abolished in 2012 after the European Court of Human Rights declared it violated Article 5 (1) of the European Convention on Human Rights; the right to liberty and security.

Although Britain stopped handing out the sentence, it did not abolish IPP retrospectively. This meant that at the time there were more than 6,000 people still serving the sentence. Twelve years on, almost 3,000 are still in prison not knowing when or if they will be released.

Britain’s Forgotten Prisoners introduces us to a handful of IPP prisoners, nearly all of whom were convicted as young adults for relatively minor crimes. The film shows us in stark terms how being put in prison with no release date is a route to insanity and hopelessness. Of the 8,711 people given an IPP, at least 90 are known to have killed themselves, and in 2023, 241 IPP prisoners were detained in secure hospitals.

In the film, the voices come thick, fast and wretched. “Imagine someone put a bag over your head and you’re in complete darkness … you’re going to go mad at some point,” says one. “The IPP sentence was designed to bury you alive,” adds another.

We meet Shaun Lloyd who was jailed for two years and nine months at the age of 18 after pushing somebody to the floor and taking their mobile phone in 2005. When his victim chased him he stopped and returned the phone. Shaun has been released and recalled a number of times for taking heroin. He became a drug addict in prison.

We see him waiting to be recalled to jail for the same crime almost 20 years later after another relapse. “I think I’ll kill myself,” he says baldly. “I’m a burden on my mum. If I was to commit suicide it would break her heart, but then the IPP would be done.”

His mother is the heroic Shirley Debono who has been fighting for Shaun and all IPP prisoners for 15 years, currently with the IPP Committee in Action campaign group. Shirley’s daughter Lisa died at six weeks and we see her visiting her grave. However painful Lisa’s death remains for her, she says it cannot compare with the grief she has experienced losing Shaun. “I want him back,” she says heartbreakingly.

Then there is Ronnie Sinclair, who served 16 years in jail on a three-year IPP tariff for stealing a flower pot at the age of 17. She is terrified of using self-service tills in case she is accused of theft and recalled to prison.

Nick Waszczuk who served 17 years for grievous bodily harm (five times over his tariff), also lives in fear of recall. “It gets to a point where just being alive hurts mentally. Is there any point in getting up every day and trying where in reality I think I’m going to die in prison?”

Greggor Grey served 19 years after receiving a tariff of four years for robbery and GBH after a pub fight. Now he hopes to hire a burger van by Edgbaston cricket ground in Birmingham, but we can see he’s struggling with his mental health. “They may not have shot them, but they’ve broken them down one by one,” his mother says of IPP prisoners. Nicolle Clarke got a tariff of 18 months for GBH and served 10 years when she was still a young mother. She is an addict, desperately trying to kick her habit and hold down a job at Iceland.

It’s not only those serving IPP sentences who are traumatised. David Blunkett, who introduced the sentence as home secretary, looks tortured when he talks about the monster he created. “I got my part of it wrong, and there are times in life when you put your hands up. I couldn’t just walk away from what had happened and I can’t now, and I will continue doing so till we put this wrong to rights.” Blunkett clearly won’t find an inner peace till he does.

Righting those wrongs would mean resentencing IPP prisoners. Those who have served more than their tariff (99%, of whom 57% had served more than 10 years above their tariff in December 2023) would be released, while the few who haven’t would be given a determinate sentence for their index crime. Last year the justice select committee recommended all IPP prisoners be resentenced.

This is when we witnessed the callous cynicism of ambitious politicians. Prison is a toxic topic for those with their eyes on the prize. Dominic Raab as justice secretary rejected the resentencing recommendation. Current justice secretary Alex Chalk has referred to IPP as “a stain” on the justice system. Despite this, he recently told the Commons that the danger of resentencing is that it would result in IPP prisoners actually being released. As for Labour, Keir Starmer sent out a firm message that resentencing should not be supported. In the months following the government’s refusal to resentence, nine IPP prisoners took their own lives.

The film’s director Martin Read tells me he only found out about IPP by accident. In 2017 he met a man who told him he was on the run. Read asked him why he was on the run, and he explained that he had missed an appointment with his probation officer and that would mean a recall to prison. Read found it hard to believe, but went to film the man where he was living. As they were filming, the police arrived to take him back to jail. It started a seven-year odyssey following Shirley Debono around the country, meeting IPP prisoners, and then having to find new subjects as one by one they were recalled to prison for minor infringements of their licence.

Britain’s Forgotten Prisoners is a profoundly distressing film. How could it be otherwise? The miracle is that it mines such depths of humanity. While the government tells us that IPP prisoners are jailed for our protection, here we get to know Shaun, Nick, Greggor, Nicolle and Ronnie, and we see them for what they are: decent, likable, damaged human beings doing their best to stave off despair and stay alive.

Read, who grew up in London on a tough council estate, related to the people he met. Most, he says, were similar to him: working class, unrooted and unqualified. He left home at 16 and found himself homeless. He then did manual labour on the railways, made pop videos and tried to break into the film world without success. Despite having no qualifications, in his 30s he was accepted on to a degree course in documentary making at Newport University and graduated with a first. While a student, he directed a film about young homeless people that won him a Royal Television Society award.

He persevered with Britain’s Forgotten Prisoners despite not getting a commission for it. It became an obsession. He took jobs to subsidise the film and just when he thought he’d finally run out of cash he was funded to finish it. “I sacrificed seven years and it’s taken a lot out of me.”

It’s not surprising. He’s forged an intense relationship with his subjects, and has ended up as a friend and therapist to them as film-maker. He says he’s often on the phone to them through the evening till 2am.

By the end of the film, Shaun and Nick have been recalled to prison while Nicolle has been ordered to wear a GPS monitoring tag. I ask him if Greggor managed to hire the burger van. No, Read says. “He was begging for mental health support and never got any. He got recalled last week. That really upset me.”

Read might have sacrificed seven years, but he’s not wasted them. With a general election a month away, he knows there is no better time to heighten awareness of the prison sentence that politicians routinely refer to as a stain on democracy while doing nothing to remove it. “This shouldn’t be happening in our country. It’s shocking. The most important thing I want is for a broadcaster to pick the film up so the public can see the injustice and cause outrage just like with the postmasters scandal. Then we might get somewhere.”

• Britain’s Forgotten Prisoners is screening at the Sheffield DocFest on 13 and 14 June.