The prime minister has insisted the government will go ahead with plans to extend the period terror suspects can be held without charge to 42 days.

Speaking at prime minister's questions, Gordon Brown declined to answer suggestions by Conservative leader David Cameron that the vote would be "an issue of confidence for his government".

Labour unrest over plans to extend the limit from the current 28 days have led to predictions of a possible defeat when the counter-terrorism bill returns to the Commons in June.

Cameron compared the situation to last week's "fiasco of the 10p tax rate", when the government offered concession in the face of a Labour rebellion.

The Tory leader asked Brown why he was pushing ahead with the plan when the director of public prosecutions, the former attorney general and the former Lord chancellor had all said 42 days was unnecessary.

And he asked the prime minister to "admit that he's going to have to make concessions on his proposal to extend detention without charge".

Brown said both opposition parties and the campaign group Liberty agreed with the principle that there would be cases where it was necessary to go beyond 28 days.

Both the police and the independent reviewer of terrorism legislation, Lord Carlisle, he said, backed the extension.

"If we have to come to the House in a period of emergency and ask for these extra powers that is not the way because that gives oxygen to terrorism," he said.

"Is is better to take the pre-emptive action now and I think the Conservative Party should be ashamed of itself for not supporting this legislation."

Brown said he believed the government had "dealt with the civil liberties arguments in this case".

Cameron said Brown was heading for "another rebellion, another back-down, another U-turn", and accused the prime minister of putting "political calculation and self-interest ahead of the right decisions and the national interest".

In an angry series of exchanges, Brown described the leader of the Opposition as a "shallow salesman".