The government has promised to investigate Conservative claims that prisoners have been released from jail against police advice.

At prime minister's questions, Tory Leader David Cameron also alleged 344 violent offenders had been freed.

Prime Minister Gordon Brown said Justice Minister Jack Straw would write to Mr Cameron on the advice issue.

More than 1,700 inmates in England and Wales have been let out early since 29 June to reduce overcrowding.

Earlier this week, the Ministry of Justice revealed six criminals released under the scheme had committed eight more crimes after being let out. A further 18 are on the run.

Objections 'over-ruled'

In the Commons, Mr Cameron repeatedly insisted Mr Brown had lost his grip on the prison service.

He said some had been released early against the advice of police and probation staff and others had gone on to commit further offences.

Mr Cameron first asked how many prisoners had been let out and how many of them were convicted of violent offences.

Mr Brown replied around 1,700 had been released and the figure did not include anybody serving a sentence for a serious violent offence.

He also said none were subject to registration under the Sex Offenders Act, had previously escaped custody, or had breached temporary release conditions.

Prime Minister Gordon Brown

According to Ministry of Justice figures 344 of those released were convicted of violence against the person.

Mr Cameron retorted: "I think you need a new briefer - 344 of those criminals were convicted of violent offences. Those are the facts."

He added: "It has also been alleged there are cases where police and probation staff objected to the release of individual criminals because of the risk they posed but they were over-ruled.

"Has that happened?"

Mr Cameron later said the over-ruling of police and probation staff had been revealed by Harry Fletcher, one of the leaders of the National Association of Probation Officers.

He added: "We said at the time that nobody who was serving a sentence for a serious violent offence would be released.

"We made it absolutely clear at the time and it is disingenuous for you now to come back and say the opposite."

He said the claim police and probation staff had been over-ruled would be investigated and Justice Minister Jack Straw would write to Mr Cameron on the issue.

Mr Cameron also urged Mr Brown to apologise to the victims of prisoners who had been released early and asked him to confirm the scheme would not become permanent.

Mr Brown said: "We will continue to review it. Early release is not something that happened just under the Labour government. It happened on two major occasions under the Conservative government.

"Each person released was assessed against the criteria that I've read out.

"As for anybody where there has been any offences committed - yes, I regret if anything has happened.

"But let's look at the evidence before us before we draw conclusions."