The House of Commons votes today on a proposal that should outrage all its members bar one. The plan to allow police to hold terror suspects for six weeks without charge should offend those who value their civil liberties because it panders to the politics of fear, because it imperils individual freedom and because it is unnecessary. But it should also offend those who truly believe that the police need greater powers to combat terrorism. It will not create those powers. On the contrary, the “safeguards” introduced in haste to head off a potential Commons mutiny have rendered this measure unworkable in practice and inimical to justice.

The only MP, therefore, who stands to gain from a “yes” vote is the member for Kirkcaldy & Cowdenbeath, Gordon Brown. This proposal long ago stopped being an attempt to facilitate the prosecution of terrorists. It is about securing the position of the Prime Minister.

So much attention has been paid to the 42-day plan as a trial of Mr Brown's strength that MPs and the public are in danger of missing what it would mean in practice. Under existing law, police need a series of warrants from district and then High Court judges to detain terror suspects without charge for up to 28 days. To hold a suspect beyond that under the new law the Home Secretary would be expected to declare a “grave and exceptional terrorist threat” and seek parliamentary approval within seven days, recalling Parliament if necessary.

In principle, police would use the extra time to decrypt, translate and analyse evidence that the Prime Minister insists is so complex that the 28-day deadline will soon be obsolete. In practice, no terror suspect has yet had to be released for want of time to prepare charges, and the Director of Public Prosecutions has said that he sees no reason to extend the 28-day limit. But even these grounds for resisting 42 days have been superseded by others arising from the Government's rearguard action to defend the plan.

MPs have been promised detailed briefings to help them to assess each request by the Home Secretary. It is hard to imagine the security services releasing the necessary information for a proper judgment, on whatever terms. But if they did, and Parliament upheld a request, it would have voted on the evidence and thereby jeopardised the suspect's chances of a fair trial. And if they did not, this vaunted parliamentary scrutiny would be little more than a charade.

This way chaos looms. Senior police officers who supported an extension of the 28-day limit on practical grounds now see the 42-day plan as an object lesson in impracticality. Civil libertarians note, rightly, that the “grave and exceptional” threat to be invoked by the Home Secretary could exist anywhere in the world and that the phrase is not, in fact, a legal precondition for extending detention without charge.

The vital democratic principle that a prisoner has the right to know why he is being held is at risk of being diluted for the sake of a prosecutor- ial process whose need is not proven and which, quite simply, cannot work. The 42-day plan attacks precisely the freedoms that Britain stands for. Most MPs supporting it will do so grudgingly at best, to give their beleaguered leader a short-lived tactical victory. Shame on them. Habeas corpus is worth more.