Leading article: Win or lose, the Prime Minister has surrendered to the politics of terror

Of all the fights that Gordon Brown has become embroiled in since becoming Prime Minister, the protracted guerrilla war with MPs over detention without trial is surely the most unnecessary and misguided. It is also the one where the position he has taken seems most at variance with his better instincts.

Mr Brown's first weeks at Number 10 were marked by a refreshing retreat from the harsh language of the "war on terror". In appointing Jacqui Smith as Home Secretary, he appeared to be signalling a calmer, more reasoned, approach to an issue that was about more than law enforcement alone. Determined and measured, their joint response to the – mercifully failed – terrorist attacks in London and Glasgow drew praise from many quarters.

Yet within months we were back in the Blair era of fearfulness. The 28 days of detention without charge for suspected terrorists – the compromise forced on Tony Blair – was suddenly not enough again to tackle the terrorist threat at our gates. Without offering any more cogent explanation than his predecessor, Mr Brown demanded 42 days; still less than the 56 days Mr Blair had wanted, but more than MPs had been prepared to support before.

We believed then, and we believe now, that any extension of the period of detention without charge is quite wrong, both in principle and in practice. It is wrong in principle for simple reasons of common justice. Anyone who is arrested is entitled to know why, and what evidence there is against them. That is an elementary right and one of the most fundamental characteristics of a civilised country. The day on which habeas corpus was effectively done away with was a day on which the quality of our justice was diminished, no matter how many supposed safeguards were introduced.

The longer detention period is wrong in practice because no one has yet demonstrated – nor come close to doing so – that national security has been jeopardised for want of even a day's investigation. Pressure for an extension has come exclusively from senior police officers – an institution where, hardly uniquely, work tends to expand to fit the time available. In the unlikely event that 28 days proved insufficient, a one-off application could surely be made before a court.

As it is, we have not only civil liberties groups, but the Director of Public Prosecutions (Sir Ken Macdonald), the former Attorney General, Lord Goldsmith, the Security Minister, (Admiral Lord West, in an unguarded moment), and a notably hard-line Shadow Home Secretary, all arguing against 42 days. The head of MI5 has also declined to support an extension, while a former Prime Minister, Sir John Major, said that it could actually encourage terrorist recruitment. Even the Home Secretary, whose advocacy of the 42 days to Labour MPs was highly praised – and who will be responsible for rescuing the Prime Minister if the vote goes his way tonight – has been selling the provision as something hypothetical, to be used only in an absolute emergency. In other words, MPs should approve the extension, safe in the knowledge that it will probably never be used. If this is the best argument that the Home Secretary can make, why on earth is Mr Brown apparently so determined to stake his political life on it?

On the eve of the vote, ministers were understandably wary of appearing too confident, while insisting that defeat would not inflict a mortal blow. While it might not precipitate a confidence vote, though, it would weaken an already debilitated Prime Minister. And the truth is that he would have only his own misjudgement to blame.