The Parole Board has come under fire for being too cautious after figures showed a dramatic drop in the number of prisoners being granted parole.

Many of those denied parole could be freed without any risk to the public, the Home Office study found.

In a separate report on the parole system, also published on Thursday, the National Audit Office warned delays in releasing prisoners cost an estimated �2m in 1998-99.

The prison population has increased by 25,000 over the last 10 years to its present figure of 65,000, and it is predicted to keep rising for the foreseeable future.

In 1998-9 nearly 7,000 prisoners were eligible for parole, compared with just over 5,000 in 1996-7, but just 40% of applications were accepted.

Risk to public

The 1991 Criminal Justice Act was expected to increase the number of prisoners released on parole requiring the Parole Board to put most emphasis on avoiding any risk of a further offence, balanced against the rehabilitation effects of early release.

But the Home Office study found the number paroled had actually fallen from 70% before the Act to 48%, while about nine out of ten prisoners now had conditions attached to their parole, compared with about 50% before the Act.

It also found Parole Board members often overestimated how great the risk of inmates reoffending was.

Only half the prisoners in the sample of cases who were judged to be a low risk of reconviction were granted parole.

'Extremely disturbing'

Paul Cavadino, director of policy at the National Association for the Care and Resettlement of Offenders (Nacro), said the findings were extremely disturbing".

He said: "The evidence suggests that the Parole Board is being over-cautious, often judging that inmates pose a far greater risk than they do in reality."

The study by the National Audit Office also pointed to flaws in the way parole was being administered.

Delays processing applications put a strain on prison accommodation and could cost about �450 a week, it found.

Head of the National Audit Office Sir John Bourn said: "Parole is an important point in the process of rehabilitating offenders.

"It therefore needs to be integrated into the programme of action - including training and education - agreed with the prisoner to prepare them for their eventual release."