Prisoners who have completed their minimum sentence should not have to prove it is safe to release them before leaving jail, a parole chief has said.
New Parole Board chairman Professor Nick Hardwick said some inmates on indefinite sentences are "festering" in prison because they cannot meet the requirements of proving they no longer pose a danger to the public.
He was speaking about Inmates on Imprisonment for Public Protection, introduced by Labour in 2003 for criminals whose crimes were not serious enough for a life sentence but who were still considered a danger to the public.
It was originally estimated that 900 serious violent and sexual offenders would be subject to them but, instead, the number swelled to 6,000, some for relatively minor offences.
Ken Clarke abolished the indeterminate sentences as Justice Secretary in 2012 - calling them a "stain" on the justice system. But in March this year, 3,347 remained imprisoned despite having served their minimum terms.
Prof Hardwick said that procedural delays, problems accessing offending behaviour courses and finding suitable accommodation meant prisoners found it difficult to meet the threshold.
He told the BBC: "Some of them are stuck, festering, in prison long after the punishment part of the sentence," he told the BBC's Today programme.
"Once it gets to that point, they stop making progress and they start going backwards.
"So this is, I think, a blot on the justice system and I'm very keen we can do something about it."
He wants to be allowed to change the criteria prisoners must meet before they can be released.
Prof Hardwick, who wants to be put in charge prisoner release criteria, said: "It's certainly an effective way of reducing some of the pressure on the prison system, which I think everybody is agreed needs to happen."
He went on: "It's a real problem, I think, if people are in prison not because of what they have done but because of what they might do."
Additional reporting by the Press Association.
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