The government is facing legal action after the United Nations concluded prisoners trapped on indefinite jail terms are being unlawfully detained, can reveal.

The UN Working Group on Arbitrary Detention called for Britain to urgently act in a scathing legal opinion, issued last month, which found five men handed Imprisonment for Public Protection (IPP) jail terms had been wronged by the state.

The human rights watchdog said that their long incarceration beyond their original tariff amounts to arbitrary detention under international human rights law and they are entitled to compensation and other reparations.

It followed a complaint made by campaigners on behalf of the men who served a combined total of 84 years incarcerated under the controversial jail terms, which were outlawed in 2012.

Lawyers are now preparing to submit a rare habeas corpus legal challenge to the High Court, which will force the government to justify the prisoners' continued incarceration to senior judges.

In a letter before action sent to justice secretary Alex Norris today, human rights lawyer Karen Todner said the British justice system had been “shamed” by the UN’s findings.

She told it has rendered the IPP jail term “indefensible”, adding: “It’s hugely significant and very damning for the UK government to receive such a condemnation of our justice system.”

She called for the government to finally bring an end to the scandal after Andy Burnham promised to review the jail terms amid a row over the government’s early release scheme and prison overcrowding.

The prime minister has asked Mr Norris to “work through further options” to free up cells so he can keep some of most serious offenders in prison after a backlash against the early release of two of the killers of PC Andrew Harper.

Options being considered include reviewing indeterminate sentences, speeding up the deportation of foreign criminals and looking at jail terms for women.

“I think it’s about time everyone just realised that the whole procedure, the whole sentencing regime [IPP] is unlawful and unfair and it needs to stop,” Ms Todner added.

Four of the five tragic cases condemned by the UN are still incarcerated, including Leroy Douglas, who has served almost 20 years without release for street robbery of a mobile phone; Abdullahi Suleman, who is still in custody 20 years after he was handed an IPP for a laptop robbery; Wayne Williams, who has spent more than 19 years in prison without release for a 23-month jail term for attempting to injure a police officer in a fight; and Shaun Lloyd, 39, who was recently hauled back to prison for the fifth time, having served more than 12 years for two street robberies.

Sadly, Joshua Mcrae, died in prison aged 34, having served more than 16 years for a four-year tariff for grievous bodily harm.

The open-ended punishments were scrapped in 2012, but not retrospectively, leaving thousands languishing in prison without a release date for years beyond their tariff, including some for minor crimes. Once released, many find themselves trapped in a cycle of indefinite recall for minor breaches of strict licence conditions.

Despite 96 prisoners taking their own lives in custody after losing hope of ever getting out, successive governments have resisted calls to resentence those still on the jail term.

However the UN’s ruling slammed the “structural circularity” of the punishment, which means the psychological harm inflicted by the hopeless sentence is then used as evidence of further risk when prisoners are assessed by the Parole Board.

They also hit out at systemic Parole Board delays and repeated prison transfers which hamper rehabilitation.

They also found Mr Suleman is being discriminated against due to his mental health because his parole hearings have been suspended while he is in a secure hospital.

The working group said the government should treat the findings as a matter of the “highest urgency”, calling for them to “ensure a full and independent investigation of the circumstances surrounding the arbitrary deprivation of liberty of the five individuals and to take appropriate measures against those responsible for the violation of their rights”

Their legal opinion comes after the UN special rapporteur on torture accused the goverment of perpetuating human rights abuses and warned the jail term may amount to psychological torture.

In her letter, Ms Todner has given the government three days to respond to the UN’s findings, warning she intends to instruct by Alun Jones KC and Muin Boase of Great James Street Chambers to submit an application to the High Court within two weeks.

She added: "The damage caused by these sentences is internationally recognised and has been rightly condemned. While it is clearly right for the Government to seek to review the sentences of all those impacted by the cruel and inhumane IPP sentences, these cases have gone on for too long for those impacted to wait for the state to resolve a two-decade-old injustice that it itself caused.

“We will therefore be applying to the High Court on behalf of our clients to have their ongoing detention declared unlawful, in line with the United Nations Working Group’s scathing criticism of both the individual sentences and wider ongoing IPP detentions. This barbaric regime must finally cease."

A government spokesperson said: “While we cannot comment on individual cases, it is right that IPP sentences were abolished. As the IPP annual report shows, support has significantly improved for these offenders, with greater access to rehabilitation and mental health support. We are now looking at what more can be done.”