The hidden price of providing support to prisoners' families could push the bill higher. Concern about the frustration that delay causes inmates and their families, as well as the escalating cost, has prompted calls for extra resources and a review of the parole system.
The backlog is said to be adding to tensions in some jails, which are already reporting increased problems because of the crackdown on prisoners' home leave and privileges.
The National Association of Probation Officers (Napo) estimates that up to 700 prisoners are experiencing delays - some up to 12 weeks - and that this will rise to about 2,000 by the autumn unless action is taken. About 65 per cent of parole applicationsare successful, so that about 455 prisoners will be running up unneccessary jail bills of about £460 a week - running into millions of pounds.
The prison service accepts there is a limited problem but says it only involves about 200 prisoners. "There are some delays because a number of prisons incorrectly processed some cases," a spokeswoman said.
But yesterday, Harry Fletcher, assistant general secretary of Napo, said that reports of a hold-up in the system were coming in from all over the country and that it was worsening daily.
One probation officer from a prison in the south of England, where inmates are experiencing a 2-3 month delay, said: "Frustration and anger run high. No official explanation seems forthcoming."
Parole system changes in October 1992 meant that all those sentenced to four years or more had to apply for discretionary parole 26 weeks before their first eligibility date, halfway through their sentence.
They are interviewed by a parole board member, their cases considered by a parole panel, and, after a final Home Office review, prisoners are supposed to be told whether they have qualified, three weeks before their release date. In one prison in the South-west, inmates who had applied for parole last autumn have still not been interviewed. In another prison, in the south, inmates with release dates in December and January have not yet had their cases considered by a panel.
Probation officers say that the rise in the prison population - now at 49,675 - coupled with a decision by ministers that the final review can be taken only be senior civil servants, has meant there is now "a serious backlog".
Mr Fletcher said: "Each time a prisoner serves longer than they need, the sense of injustice mounts and the costs to the taxpayer accelerates.
"By October, the additional costs in prison warehousing will exceed £11m. Ministers need to urgently review the resourcing of the parole unit. Parole delay is a dangerous addition to the consequences of their new austere rules and regulations."