Both the Conservatives and the Liberal Democrats - in a far cry from the political consensus ministers originally pledged to seek - said her plans risked alienating Muslims and driving people to terrorism.

Ms Smith said she was still hopeful of winning cross-party agreement on the need to increase the pre-charge questioning limit from 28 days to 42. She added that she would get the legislation passed despite the threat of a backbench rebellion.

Asked what flashpoints would arise in the Bill's passage through Parliament, Ms Smith said: "I think that we will get this legislation on to the Statute Book. I don't envisage flashpoints as such."

Also unveiled in a new Counter Terrorism Bill were plans for a criminal offence of "communicating, publishing or eliciting" information about service personnel. Gathering details about members of the armed forces will carry up to 10 years' imprisonment if there was suspicion of a plot to place them in danger.

The issue of pre-charge detention led to Tony Blair's first defeat in the Commons in November 2005, when he sought a 90-day limit. A survey of Labour MPs by the Independent newspaper last month indicated that enough Labour MPs would vote against the new 42-day proposals for a Government defeat in the Commons.

Shadow home secretary David Davis said: "Last week, Jacqui Smith rightly highlighted the danger of radicalisation. Today, her fixation with extending pre-charge detention risks serving as a recruiting sergeant for terrorism."

He added: "The Government has not been able to present a shred of evidence to justify extending pre-charge detention, and there is now a range of evidence pointing the other way."

Liberal Democrat leader Nick Clegg said: "The obsession with 42 days is undermining, not supporting, the battle against terrorism. Ministers are taking their reliance on hypothetical examples to new lengths as they attempt to cover up for the complete lack of genuine evidence for such a move.

Director of human rights group Liberty, Shami Chakrabarti, urged the Government to drop the policy, saying: "Despite ministerial promises of exceptional circumstances and so-called safeguards, the reality of this Bill is an on-off button for six weeks' detention without charge."