The Liberal Democrats have made clear there is no prospect of them supporting a key plank of the government's anti-terrorism legislation.

Ministers want to allow police officers to hold a terrorism suspect for up to 30 days without charge.

Speaking at prime minister's questions in the Commons, Charles Kennedy called the proposal "imprisonment without charge for 90 days".

He seized on comments from the attorney general's spokesman earlier this week that the government's most senior law officer was not convinced of the plan to detain suspects for that long.

"It seems clear that no consensus exists within government for this change," the Lib Dem leader said.

"What is the attorney general's view?" he asked.

"Or is this just going to be another occasion when the attorney general's internal views go unreported to this House?"

Tony Blair replied that Lord Goldsmith's stance had been "extensively reported" in the press and accepted that there will be differences within "political parties and across this House".

"What I have got to try to do, and I'm going to try to do it in a spirit of reaching consensus on it, is to take the proposals that have been made by the police, and lay them out before people and then let us have a debate about whether they are a sensible way to proceed or not," the prime minister said.

But Kennedy claimed there were alternative ways of dealing with the terrorist threat that need not involve breaching civil liberties.

He suggested ministers make greater use of the new offence of acts preparatory to terrorism, contained in the Terrorism Bill.

"Why is it he remains absolutely wedded to this proposal for 90 days?" Kennedy asked.

"Surely it is wrong and surely he is going to have to back down?"

Blair said it was "no great mystery" why he backed the plan.

He said a carefully considered case had been put forward by the Association of Chief Police Officers.

"I have to say I find it completely compelling," the prime minister claimed.

He argued that the "particular nature of this type of terrorism means the police will have to arrest people relatively early in the conspiracy... and that is why the police need more time to back up any charges".

Blair added that the legislation is "consistent with our obligation to protect the people of this country".