Brown faces first defeat over 42-day detention

In an attempt to win over critics the Government said MPs would be allowed to vote on each decision to hold a suspect beyond the current 28 days.

But, with the vote taking up to 30 days to organise, suspects would almost certainly have been released or charged so the safeguard would be pointless.

A parliamentary debate could also prejudice a fair trial if it revealed sensitive evidence.

Labour back-benchers are deeply unhappy with the idea. And, with opposition MPs and peers certain to object, Mr Brown faces an uphill task to avoid a humbling first defeat as Premier early in 2008.

An attempt to introduce a 90-day limit led to Tony Blair's first defeat in the Commons in November 2005.

Under the plans, Home Secretary Jacqui Smith will be able to immediately extend the current 28-day limit to 42 days if a joint report by a chief constable and the Director of Public Prosecutions supports such a move.

It would take place only in "exceptional circumstances", such as an emergency in which multiple-terror plots were on the brink of fruition.

The Commons and Lords would then have to approve the extension within 30 days.

Miss Smith said: "This is about legislating now for a risk that I am clear does exist, chief constables are clear exists and the independent reviewer of terrorism legislation is also clear exists.

"If it did need to be used it would be in exceptional circumstances and in a way where there was a proven need for it. It is not something we are expecting to become mundane."

Sue Hemming, head of the Crown Prosecution Service's counter terrorism division, said: "We have real concerns that, if there is an open Parliamentary debate about particular individuals, what would be said would become public before that individual went to trial and there may be an issue over fair trial."

Labour backbenchers gave clear indication of a revolt.

David Winnick, who led opposition to the earlier proposal for 90- day detention, said: "No evidence has been produced in my view - and in the view of a good number of other people who have taken a close interest in this matter - that any extension is necessary."

The Tories and Liberal Democrats also condemned the plans, which the Home Secretary will be called to explain at a meeting of the Home Affairs Committee next week.