Hundreds of convicts who could be released are being held in jails because the Government has failed to pay for enough drug treatment and rehabilitation courses, a leading prison governor warned yesterday.
Paul Tidball, the head of the Prison Governors Association, said there was a "mismatch" between prison places and the resources needed to "make sentences work".
His comments follow a criminal justice study commissioned by the Liberal Democrats, which found that more than 1,700 inmates had served longer than their minimum tariffs under the new indeterminate sentence for public protection (IPP), brought in by Labour four years ago.
More than a third of the 5,300 people jailed under the IPP scheme have been unable to attend an offending behaviour or drug treatment programme while in custody. Only 60 IPP offenders have ever been released.
Mr Tidball said of the IPP policy: "There is no doubt that there is a mismatch between demand and resources. The Government has over the years promoted policies which mean considerably more use of imprisonment and yet has been unwilling, and now is unable, to provide the resources to make the sentences work."
He added: "Though the public will be relieved to know there is no doubt that high-risk prisoners will be held in custody for as long as it takes for them to be properly assessed and rehabilitated, they will be less pleased that a shortage of resources means offenders are being held longer than necessary at considerable expense to the taxpayer."
Offenders given an IPP are told they must serve a minimum term, after which the Parole Board will decide if they can be released. If they are not freed, they remain in jail indefinitely with regular two-year reviews of their case by the board.
Figures obtained by the Liberal Democrats show that IPP sentences are being used twice as often as ministers had intended and are helping to drive up the prison population, which is already at a record high. Last year, the Government was forced to change the law to stop the courts using IPP sentences for less serious crimes.
Andrew Stunell, the Liberal Democrat MP for Hazel Grove, who carried out the study, said: "These prisoners are clogging up prisons at great public cost simply because the Government brought in completely unworkable sentencing rules. IPP prisoners sentenced under the original rules are stuck inside with no prospect of getting the treatment they must have before release."
The concerns raised by Mr Tidball and Mr Stunell were echoed by Juliet Lyon, director of the Prison Reform Trust. She said: "For years to come, the IPP sentence will be a textbook example of a bad idea hastily formed to sound tough on law and order, and then implemented disastrously.
Since April 2005, this ferocious runaway sentence has netted more than 5,000 people and accounts for around 6 per cent of today's prison population. "It is likely that many of the 1,700 IPP prisoners being held beyond the minimum tariff set by the courts are there because they have had no opportunity to demonstrate whether they pose a risk if released.
"Even where offending behaviour courses are available, many people with low literacy levels, with learning difficulties and most people with learning disabilities are barred from entry. Ministers cannot abandon these people in a maze with no exit, or think the problem will go away if they just ignore it."
Earlier this year, the Chief Inspector of Prisons, Dame Anne Owers, said jails in England and Wales were "increasingly fractious", with more inmates on longer sentences feeling that they had "little to lose". In her annual report, she added that disturbances in jails had been contained so far but identified "real risks" of a loss of control in the future.
A Ministry of Justice spokesperson said: "We have put in place a number of measures to manage short-tariff IPP prisoners more effectively. A new simplified parole process was implemented on 1 April, this has agreed end to end targets allowing for performance to be monitored for each element of the process. This offers clear lines of accountability and hold agencies to account for their performance."
She added: "Further funding of £3m was made available to prisons in 2008/09 specifically for the management of indeterminate sentence prisoners which has primarily been used on interventions."