Gordon Brown will spend this weekend telephoning Labour MPs to rally support for the extension of pre-charge detention to 42 days in next Wednesday's crucial Commons vote.
The Prime Minister is planning to be at Chequers, where his allies said he would be contacting wavering MPs to lobby them for their support in the vote, which could still be on a knife edge.
But there were signs the rebellion could be growing. Paul Farrelly, a Labour backbencher, said he would join rebels voting against 42 days, despite being among those who backed Tony Blair's failed attempt to extend the limit to 90 days in 2005.
He said he had been persuaded by expert opinion, such as that of the Director of Public Prosecutions, that an extension was not needed and in fact could make matters worse.
And he also criticised the policy as a "political own goal" that betrayed Mr Brown's pledge of a "fresh approach" to government when he succeeded Mr Blair almost a year ago.
David Winnick, who led opposition to the 90-day proposal, said that the vote would be "very close" but that he was increasingly optimistic ministers would be defeated.
"The Government is increasingly isolated on this issue. It stands alone with the Metropolitan Police Commissioner and his staff in wanting it," he said.
The rebels, led by former health secretary Frank Dobson, are resisting warnings by the whips that they will plunge the Government into crisis if Mr Brown is defeated on the issue. Mr Brown has privately made it clear he does not regard it as a resigning issue if he loses.
The stakes could not be higher for Mr Brown, whose leadership could be brought under question if he loses the vote after offering a range of compromises on the proposal to extend police powers to hold terrorist suspects without charge from 28 days to 42 days.
The Tory leader, David Cameron, could face criticism from his own side for insisting on opposing the Government. Ann Widdecombe, the former home office minister, has rejected the official Tory line and will vote with the Government.
Sir John Major, the former prime minister, also attacked Mr Brown's policy, saying there was no evidence to suggest that an extended detention period would have prevented past atrocities or guard against future terrorist attacks.
But the Prime Minister's official spokesman dismissed Sir John's criticism as out of touch. "A lot has changed in the past 10 years – the terror threat is a lot different," he said.