Lord Falconer has changed his mind on longer detention

He told BBC Radio 4's World at One the measure was "not necessary and not needed to fight terrorism".

Lord Falconer, who backed Tony Blair's attempt to extend the limit to 90 days, says he has changed his mind because suspects are being charged earlier.

He also dismissed the key safeguard offered to win over sceptics.

Lord Falconer said that allowing a vote in Parliament on any case where detention went beyond 28 days was not realistic and risked compromising any police investigation.

'Solution to the problem'?

He said an "artificial" limit of 28, 42 or 90 days for holding a terror suspect without charge was not needed if the threshold for when charges should be brought was lowered.

Usually a person is not charged unless there is a 51% chance of the case being a success, he said.

How can Parliament make a judgement about an individual investigation? Lord Falconer

Now the Director of Public Prosecutions has indicated this threshold can be lowered so that people can be charged when there is a less than 51% chance of success.

"It is, I believe, the solution to the problem," he said, because it removed the "ever increasing pressure" to push up the days for holding terror suspects without charge.

Lord Falconer said he did not agree with plans to give Parliament a vote when detention went beyond 28 days.

"I don't see that as a realistic possibility. How can Parliament make a judgement about an individual investigation?" he asked.

Lord Falconer is the latest in a series of high profile figures - including the director of public prosecutions and ex-attorney general Lord Goldsmith - to criticise the plans.

And two Parliamentary committees - the Home Affairs and the joint committee on human rights - both came out last week against extending the terror limit.

'Potential risk'

Home Secretary Jacqui Smith says that because of the increasing complexity of terrorism cases, police may need to hold someone longer than 28 days in order to "prosecute people who want to cause murder and mayhem on our streets".

But plans to extend the limit before have proved controversial and moves to extend it to 90 days in 2005 were defeated by a combination of Tory, Lib Dem and some Labour MPs.

The home secretary has said she wants to get a consensus on any extension, but there is little sign of one at the moment.

Her proposals would involve the House of Commons and the House of Lords retrospectively being asked to approve any decision to hold an individual for longer than the 28 days, up to the 42 days maximum.