The Home Secretary Jacqui Smith will set herself on a collision course with Labour rebels today by unveiling controversial new terror laws.
Ms Smith announced last month that she would seek to raise the limit that terror suspects could be detained without charge from 28 days to 42.
Her bid to get today's Counter-Terrorism Bill through Parliament could prove to be Gordon Brown's toughest challenge so far as Prime Minister.
A survey of Labour MPs by the Independent newspaper last month indicated that enough Labour MPs would vote against the Bill for a Government defeat in the Commons.
Only 34 votes are needed to defeat the plans and the poll found at least 38 MPs were vowing to oppose moves to extend the existing limits.
The Bill will see ministers return to an issue which led to Tony Blair's first defeat in the Commons in November 2005, when he sought a 90 day limit.
Under Ms Smith's proposals:
* The Home Secretary will be able to immediately extend the limit to 42 days if a joint report by a chief constable and the Director of Public Prosecutions backs the move;
* The Commons and the Lords will have to approve the extension within 30 days;
* The new limit will only be available to police for two months unless it is renewed;
* Parliament could be recalled from summer recess if a vote was required on an extension;
* Individual detentions over 28 days would need to be approved by a judge at least every seven days.
However, the way the proposed system is set up could mean suspects being held for 42 days even if Parliament eventually refused permission.
Such a scenario could arise if the Home Secretary only decided to extend the limit towards the end of the existing 28 day slot, because Parliament is only required to vote within 30 days.
In that time, a suspect could already have been charged or released without charge.
Last November there were red faces in the Home Office when security minister Lord West said that he was not convinced that more time was necessary.
In a radio interview Lord West said: "I still need to be fully convinced that we absolutely need more than 28 days."
However, within an hour after attending a meeting with Mr Brown in No 10, he emerged to issue a hasty U-turn, telling reporters: "My feeling is, yes, we need more than 28 days."
Today's Bill is also likely to include other counter-terror measures including moves to allow police to question suspects after they have been charged.
According to today's Guardian, the detailed legislation is expected to be tougher than originally trailed, with no legal definition of the seriousness of the alleged offence that could trigger an exceptional period of detention beyond the current 28 days without charge.
Ms Smith said the Government could not afford to "sit on our hands" in preparing for potential future risks - but denied she was legislating for a hypothetical situation.
"We need to legislate now for that risk in the future," she told BBC Radio 4's Today programme.
"It won't be hypothetical if and when it occurs. We are not legislating now on the basis that we are bringing it in now for something that might happen in the future; we are bringing in a position for if it becomes unhypothetical. If, unfortunately I and many other experts are right and we do need it in the future it is in place.
"We face a choice here. We can either sit on our hands, failing to recognise where there is a broad consensus that this is a risk that is growing that we might well face in the future, we can risk that happening, we can risk having to legislate in an emergency in the future, we can risk, as some people believe we should do, having to declare a national emergency in order to be able to do it, or we can, as we are proposing, legislate now - with the discussion that will be put in Parliament on the safeguards and on the circumstances in which it would be used - and have that available in the future.
"That seems to me to be a very sensible way forward."
She said there would be "considerable legal difficulties" in using the existing Civil Contingencies Act - as proposed by campaign group Liberty and the Opposition - but that those ideas had been incorporated into the proposals.Reuse content