Gordon Brown will take one of the biggest gambles of his political career today by defying Labour MPs to back him or defeat him over holding terrorist suspects for 42 days.

Although he knows that a reverse in the Commons on such a key issue would place a massive new question mark over his leadership, the Prime Minister writes in The Times that he will not give way. “I will stick to the principles I have set out and do the right thing,” he says.

With MPs returning to Westminster for the first time since the Crewe & Nantwich by-election disaster, Mr Brown is employing shock tactics ten days before the 42-day vote to warn MPs that although ministers will continue to strengthen safeguards they cannot expect another about-turn.

By doing so he is putting his authority on the line at a time when his own standing among MPs has been severely weakened by recent elections and dismal opinion polls and when whips privately admit that the votes are not yet there for the 42-day plan.

An ally of the Prime Minister said: “This is making clear that on 42 days, this is not a negotiation. He will not blink. This is not about getting the legislation through at any price. MPs need to know now that he will not give on 42 days.”

Mr Brown admits in The Times that he is rejecting advice, much of it known to be from influential back-benchers, that “I should drop or significantly water down the 42-day limit”. But having considered all the arguments he concluded that it was the right way to protect national security.

Mr Brown’s decision to issue such an unrelenting message, which will be repeated by Jacqui Smith, the Home Secretary, when she addresses the Parliamentary Labour Party tonight, is designed to show MPs and the country that although he was forced into a climbdown over the 10p tax rate abolition, Mr Brown is not prepared to bend on the principle of 42 days.

The Times also reveals today that Lord Levy, Tony Blair’s former fund-raiser, has been in secret talks with a senior Conservative over education policy, but he has not been offered or sought a role.

By taking an uncompromising line Mr Brown knows that he is risking another huge blow to his authority if, despite all the warnings and strictures from whips, Labour MPs were to combine with Conservatives and Liberal Democrats to defeat him.

The vote is now seen as Mr Brown’s next substantial hurdle if he is to avoid private calls for his departure continuing and for him to be given a breathing space to recover.

With at least 50 Labour MPs thought to be ready to vote against him or abstain, ministers know that if the vote were held today they would probably lose.

But by making concessions on the edges of the plan, warning MPs of the severe consequences for the party of a Commons defeat, and trying to win over Democratic Unionist MPs to back them, they believe that they can turn it round.

Mr Brown will not be attending tonight’s meeting of Labour MPs. Ministers assume that he is doing so because he wants it to concentrate on the Home Secretary’s defence of the detention powers rather than allow it to turn into a “crisis meeting” and inquest into Crewe, which might be encouraged by his presence.

Ministers will outline a series of concessions on safeguards over the next few days designed to give wavering MPs good reasons for drawing back from revolt over the 42-day plan.

Jack Straw, the Justice Secretary, indicated yesterday that the period by which Parliament has to approve the use of detention powers will be cut from the existing 30 days to something much shorter. This could be as little as a week and would mean Parliament having to be recalled during long recesses if need be.

Ministers are also proposing to cut from 60 days to 30 days the period during which the emergency powers can be used. Perhaps MPs’ biggest worry is the nature of the “exceptional circumstances” under which the powers can be used. Ministers are expected to give more details this week.

Mr Brown writes today that with those parliamentary, judicial and independent protections in place, “I believe Parliament should take the right decision for national security.”

Mr Straw and Geoff Hoon, the Chief Whip, who have strongly favoured concessions on the detail to get the main 42-day plan through, suggested yesterday that the Government could eventually win the day.

Whatever happens in the Commons, Mr Brown has been warned that his proposals could be defeated in the Lords.

Lord Goldsmith, who was Attorney-General before Mr Brown took over, gave warning that the move would be an attack on the country’s “fundamental freedoms”.

He said: “Some supporters of 42 days say we have to take this step to protect our values and our way of life which terrorists threaten to destroy.

“But we start ourselves to destroy these values and the very basis of the free society which our ancestors fought hard to create if we readily give away critical liberties, such as the right we all have not to be arbitrarily held without charge.

“And there can be no mistake that extending the period suspects can be held without charge is a very serious incursion on our fundamental freedoms,” Lord Goldsmith added.

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