Plans to detain terrorist suspects for up to 42 days without charge suffered a fresh setback after the controversial measure was described as unnecessary by the Director of Public Prosecutions.
Sir Ken Macdonald made clear his scepticism about the move and disclosed that no one had been held for more than 14 days since last summer. And Lord Goldsmith, the former attorney general, warned the extension could backfire by encouraging angry young Muslim men to turn to violence.
Their intervention will embolden Labour MPs who are threatening to vote against moves to raise the detention period from 28 days to 42 days. The vote is likely to be held in June.
Government whips fear as many as 50 rebel backbenchers are preparing to oppose the increase, enough to demolish Gordon Brown's majority of 66. Efforts by Home Office ministers to talk dissidents round are hampered by the febrile atmosphere in the Parliamentary Labour Party.
Sir Ken undermined the Government's case yesterday when he told MPs he believed the present detention period was sufficient and was likely to remain so.
He put himself at odds with Sir Ian Blair, the Metropolitan Police Commissioner, who told the Commons' Counter-Terrorism Bill standing committee that suspects "suddenly emerge from left field" and had to be arrested at a very early stage.
Sir Ian said: "We have reached a point where at 28 days we feel sooner or later – and maybe sooner – something is going to happen to make that insufficient."
Sir Ken said: "Our experience is that we've managed comfortably with 28 days. We have therefore not asked for an increase to 42 days."