About 550 non-violent and non-sexual offenders will be automatically freed halfway through their sentences, instead of having to wait until the two-thirds point.

The Ministry of Justice (MoJ) indicated that after the initial batch were freed over 14 months, there could be further releases under the same measures.

Jails in England and Wales have been instructed to let out eligible offenders from June 9, and were warned by Prison Service HQ that failing to do so would amount to "unlawful detention".

Former Lord Chief Justice Lord Woolf warned of a summer of "small scale rioting" due to overcrowding in an interview with the Daily Telegraph. He says tensions could be exacerbated by hot weather.

He told the newspaper: "The present situation is extremely worrying. I don't think prisons will blow up tomorrow or next week but there is certainly a danger of that."

Shadow justice secretary Nick Herbert described such early release schemes as "underhand", although the MoJ pointed out the plans went through Parliament unopposed.

Tory Mr Herbert said: "Releasing prisoners early when they have only served half of their sentence undermines trust in the criminal justice system, even more so when the Government has introduced such underhand measures because it has failed to provide adequate prisons capacity.

"Punishments should fit the crime and there should be honesty in sentencing so that every offender serves a minimum sentence handed down in court."

Justice Secretary Jack Straw's plan is expected to free urgently-required space in overcrowded jails, as inmate numbers reach a record 83,000 in England and Wales. The early release plan equalises the arrangements for offenders sentenced under the 1991 Criminal Justice Act with those punished under Labour's 2003 Criminal Justice Act 2003, which came into force in April 2005.