Clarke under pressure on prison early-release plan

Under the Home Office plans, prisoners could be freed, wearing an electronic tag, a full six months before they became eligible for automatic release half-way through their sentence. That would mean a prisoner sentenced to a four-year term could be released after 18 months. At present prisoners can be freed on a tag four and a half months before their automatic release date.

The Home Office insisted no decision had been taken about the proposals, being considered in response to a "spike" in prison numbers that has meant more than 100 inmates spending the night in police cells this week.

Prison reformers welcomed the proposal, but David Davis, the shadow Home Secretary, said the proposal "completely undermines the tough rhetoric spouted by the Prime Minister a couple of days ago". He added: "By releasing unsuitable prisoners early the Government will be putting the public at unnecessary extra risk as well as ensuring that prisoners are neither punished nor rehabilitated."

Mark Oaten, the Liberal Democrat Home Affairs spokesman, added: "After eight years in government, Labour's prison policy is in tatters. The length of sentences should be set to punish and rehabilitate, not as a panic measure when prisons become overcrowded."

Mr Blair's spokesman acknowledged the Home Office scheme might appear "superficially" at odds with Mr Blair's stance on crime, but said community penalties were effective and could cut rates of re-offending. "If you look at the complexity and the reality people should not get that message."

The Home Office insisted there was still spare capacity in the prison system and said extending the early release scheme was just one of several proposals being investigated. The Home Office minister Fiona Mactaggart told the BBC: "We are looking at whether we can expand the use of the existing home detention curfew scheme. There is no decision taken. It's one of the options."

Harry Fletcher, of the National Association of Probation Officers, said: "Home detention curfew has been used as a safety valve to try to control prison numbers for five years, but only six out of 10 of those eligible are actually released. The real issue is too many people being sent to prison in the first place."