The director general of the Prison Service has admitted that he does not know exactly how many inmates are on the run from open jails.

Phil Wheatley said there was no system in place to count the numbers recaptured after absconding, but such a database would now be developed.

Classed as Category D, open prisons have a more relaxed security regime.

Almost 700 offenders absconded in the year to last April from England's 15 open prisons, Mr Wheatley estimated.

Earlier this week, the Home Office, replying to a Freedom of Information Act request, said 401 of the prisoners remained at large, but the figures were compiled last May.

But Mr Wheatley said he was "embarrassed" to admit he was unable to provide an accurate up-to-date figure because there was no central database for recording numbers of recaptured prisoners.

Open prisons are often used to prepare low-risk inmates for release.

Mr Wheatley said the "vast majority" of inmates who abscond were "arrested promptly".

"I know that about three-quarters of the people at liberty unlawfully are back inside within a 12-month period," he said.

He said the number of people absconding from open prisons was at its lowest level for more than 10 years.

A central database would not help the service track missing prisoners any more efficiently, but one would be created in order to answer future queries, Mr Wheatley added.

"That will cost us some cash. We will divert it to make sure we build a system so we can respond to queries. It won't add any value in terms of are people caught or not.

"The crucial thing is that the police know who is unlawfully at large, and when they pick someone up know is that person unlawfully at large and can they be brought back into custody. That the system does perfectly."

Liberal Democrat home affairs spokesman Nick Clegg said Mr Wheatley's admission was "yet another symptom of the government's self-inflicted overcrowding crisis in our prisons".

"With chronically overcrowded prisons, it is inevitable that the wrong kind of offender ends up in open prisons and that the system simply struggles to keep a proper head count," he said.

The Probation Officers' Union, Napo, said the Home Office's policy of transferring prisoners to open jails to ease overcrowding in jails needed to be "urgently reviewed".

Napo's Harry Fletcher said overcrowding in the prison system had led to an increase in inmates being transferred to open jails since October and it would be "wise" to update IT systems.

But Juliet Lyon, director of the Prison Reform Trust, said open prisons were a vital part of the rehabilitation process of long-term offenders.

"Instead of panicking about absconds from open prisons... or berating the Prison Service which, despite gross overcrowding, has been able to reduce absconds and cut escapes from closed institutions, we should all demand to know what the Home Secretary is doing about appalling reconviction rates," she said.

She said those were running at over two in three of all released prisoners and well over three-quarters of young offenders.

Meanwhile, police hunting two convicted murderers who absconded from an open jail released photos of the pair.

Jason Croft, also known as Jason Fox, 28, from Salford and Michael Nixon, also 28, of Blackley, Manchester, left Sudbury Prison in Derbyshire last year.