The Government's plan to extend from 28 to 42 days the period for which terrorism suspects can be held before charging them has become more about the retention of the Prime Minister than the detention of terrorists. With a familiar display of stubborness, Gordon Brown has made his 42-day proposal a needless test of virility: both of himself and now also - to the evident alarm of his Labour colleagues - of his party.

That the Home Secretary, Jacqui Smith, yesterday tabled a series of concessions is a sign not of a government that is listening and learning, but of a government willing to bring the bartering of the souk to the backrooms of Westminster in order to cut a deal. Either the renegades in the Labour Party believe the extension to 42 days to have merit as a matter of principle, whatever the fine-tuning Mr Brown introduces, or they do not.

To be seduced by such tinkering reminds you of the story of George Bernard Shaw asking a lady seated next to him at a grand dinner if she would sleep with a man for a million pounds. When she replied coyly that it would depend on how handsome he was, Shaw asked if she would sleep with a man for five shillings. “What do you take me for?” stammered the woman. “We have already established that,” Shaw replied. “We are now just haggling over the price.” The principle here is simple enough: you are either in favour of 42 days, or you are not. If there is one thing worse than wanting an extension of detention powers to 42 days, it is getting one that is so hollowed out as to be the worst of all worlds. We do not employ our politicians to pass gesture legislation.

As for the case for an extension, there remains nothing to jolt our wariness. In this newspaper on Monday the Prime Minister argued for an extension because investigations today are broader and, so, longer. Downing Street claims to be making a principled stand, but its argument is a practical one: police investigations may need more time to examine potential evidence. But, if they have more time, they are more likely to use it - and at the expense of an individual's freedom. The real principle here is the protection of hard-won civil liberties against the politics of fear.

Champions for an extension are a lonely band. It is not just that the plan plants Labour to the right of both the Liberal Democrats and the Conservatives, it is that the Director of Public Prosecutions, the former Attorney-General, and Parliament's Joint Committee on Human Rights see no need to shift from the current 28 days. Indeed, since the limit was raised to 28 days in 2006, only 11 suspects have been held without charge for longer than 14 days. The Civil Contingencies Act - passed by this Government - already gives Mr Brown the emergency powers he seeks, but with built-in safeguards. Worse, by alienating local Muslim communities, the new law risks inflaming the very problems it aims to douse.

Since 2001, the Labour Government has not shied from the fight. So there is neither a principled reason nor a political advantage to the stance it is taking now. A handy rule for governments, whether setting detention powers or road speed limits, is to settle on a figure that reaps the greatest benefits with the least corrosion of people's liberties. Having settled on 28 days, the onus is on the Government now to make a convincing case for altering it. Having failed to make one, the present ceiling should be left to stand.