Justice Secretary Liz Truss must “get a grip” on the backlog of inmates being held beyond their sentence, the chief inspector of prisons has said.
Peter Clarke said it was “completely unjust” that offenders serving Imprisonment for Public Protection (IPP) terms were “languishing in jail”.
He warned that IPP sentences, abolished in 2012, were having a serious effect on prisoners’ mental health.
The Ministry of Justice said a new unit had been set up to tackle the problem.
More than 3,800 prisoners in England and Wales are serving indeterminate IPP sentences, designed to protect the public.
Of those, 500 should be let go, former Justice Secretary Michael Gove said, when he delivered the annual Longford lecture in memory of prison reformer Lord Longford.
Mr Gove said executive clemency should be granted to release prisoners who had served far longer than the tariff for their offence and had now – after multiple parole reviews – served even longer than the maximum determinate sentence for that offence.
Speaking to the BBC’s Today programme, Mr Clarke said Mr Gove was the latest in a line of secretaries of state who had pointed out flaws in the system.
The prisons inspectorate identified problems with IPP sentences eight years ago, yet little had been done since and progress was “painfully slow”, Mr Clarke said.
He added: “This should be addressed as a matter of urgency, and it’s not just a case of resources – there have been failings and blockages in the prison service, in the probation service and the parole board.
“And we suggest that the only person who’s got the authority to get a grip on the way things happen – it may mean policy changes…is the secretary of state [Liz Truss].”
On prison visits, he said inmates – including one who was seven-and-a-half years over his tariff – told him they felt “trapped in the system” and unable to prove that they were no longer a risk to the public.
One IPP prisoner, James Ward, told the BBC he feared he would never get out. He is in his 11th year in prison after being given a 10-month sentence for arson.