The former Conservative leader said Gordon Brown's proposal to extend the detention without trial period would be more likely to boost terrorist recruitment than tackle the security threat to Britain.

Writing in The Times newspaper, Sir John also said the Labour Government's response to terrorism had eroded civil liberties and that the invasion of Iraq had damaged Britain's reputation overseas.

His comments are a further blow to the Government's efforts to win support for the 42-day increase ahead of a vote in the Commons next week.

The criticism comes after the joint committee on human rights said it remained opposed to the extension, despite a series of concessions outlined by Home Secretary Jacqui Smith in a last-ditch bid to head off a highly-damaging Labour backbench revolt.

In one of his fiercest attacks on Labour since he was defeated in the General Election in 1997, Sir John described the case for detaining terrorism suspects for 42 days without charge as bogus and little more than scaremongering.

"I don't believe that sacrifice of due process can be justified. If we are seen to defend our own values in a manner that does violence to them, then we run the risk of losing those values. Even worse, if our own standards fall, it will serve to recruit terrorists more effectively than their own propaganda could ever hope to," he said.

"The Government has introduced measures to protect against terrorism. These go beyond anything contemplated when Britain faced far more regular - and no less violent - assaults from the IRA. The justification of these has sometimes come close to scaremongering."

Sir John said there was no evidence to suggest that an extended detention period would have prevented past atrocities or guard against future terrorist attacks. "No example has yet been given of why the police need more than 28 days to frame a charge. This is a slippery slope. Assertions that it 'might be useful' simply will not do. If we are to curtail the liberty of the individual, we must have more certainty than that."

He argued that long-held British rights were being damaged by a Government that embellished the case for going to war in Iraq and was complicit in the rendition of suspects to Guantanamo Bay. "The Government's legislation to permit 42 days pre-charge detention brings to the fore the wider question of civil liberties. In their response to the security threat ministers have dragged us ever closer to a society in which ancient rights are seriously damaged. I doubt this is the Government's intention, but it is the effect. It began with Iraq."