The government has failed to make the case for 42 days

So must it be 42 days? Has the government stifled its backbench rebellion, leaving the Lords to rescue British liberties, as they did eight centuries ago at Runnymede? Or can Labour MPs tear the fair maid of conscience from the seductive arms of expediency, and tell Gordon Brown to get lost?

Brown and his ministers claim that "terrorism" - code for militant Islam - poses an unprecedented threat to western civilisation. Why then do they persist in undermining a pillar of that civilisation, freedom under the law? Why keep giving Osama bin Laden new feathers to put in his turban?

The bill installing 42-day detention without charge goes to the Commons next week, with the full weight of government whipping behind it. It is equalled in almost no other free country and backed by almost no one in Britain's judicial or security establishments. Opponents or known sceptics embrace the former lord chancellor, the former attorney general, the security minister, the director of public prosecution, numerous police chiefs including London's and, so it is said, the leadership of MI5. In an unguarded moment, the whips even murmured that Brown regards it as an inconvenience "inherited from Tony Blair".

The only support has come from an eccentric Liberal Democrat peer, Lord Carlile, and the former Scotland Yard counter-terrorism chief, Peter Clarke. The latter's defence of the bill in yesterday's Daily Telegraph amounted to little more than a belief that 42 days would be useful to his colleagues, and a complaint that it had all become politicised.

Hostility to the bill has forced Downing Street into precisely the bind that a "listening" government hates. What began as a police try-on of weak Labour home secretaries has turned into "a principled stand in defence of national security". This has then mutated into a talisman of the prime minister's steadiness under fire.

Brown and his staff have been debating clear choices on the bill in recent weeks. Do they hold fast and fight to the death? Do they cut and run with good grace? Or do they twist, slice, spin, threaten and bribe? The predicamentis hardly new. If the Downing Street walls could sing, they could do this one in harmony. Dissidents have even been asked to propose safeguarding amendments, which have been written for them.

The mooted changes have turned the detention parts of the bill into a dog's breakfast, akin to other emergency powers and civil contingency laws. Brown and his home secretary, Jacqui Smith, have been reduced to declaring the bill truly and exceptionally unusual, whatever that means, and declaring that 42 days are just right, as they previously said of 14, 28 and 90 days. Such legislation is either so permeated with executive sleight of hand as to be outrageous, or so emaciated by judicial and parliamentary oversight as to be unusable.

The result has blighted what should be a clear and simple emergency measure. Detention beyond 28 days is to be allowed "only where there is a specific terrorist incident or threat", but in that case there would surely be enough evidence to lay some charge - at least a holding charge - which is all the police need to extend detention anyway.

As it is, a judge will have to approve the extension - on the basis of some evidence or other - and his decision will have to be approved by parliament within (we are now told) a week. Smith explained yesterday that this meant her asking the Commons to "test the circumstances of the threat" in each case against the law. How can this be if MPs cannot be given any material that would prejudice a future trial?

Yet if MPs refuse to approve the extension order, Smith says that "the existing 28-day limit would stand", which presumably means the suspect being instantly released. This is worse than executive and political pollution of the judicial process; it is unimaginable in practice. Unimaginable legislation is bad legislation.

The one case constantly cited (not least by Brown and Clarke) is the Dhiren Barot terrorist plot of 2004. To get a charge together within the then limit of 14 days, said Clarke, "police officers spent a fortnight sleeping on their office floor ... in a race against time". They had to handle "270 computers, 2,000 disks and more than 8,000 other exhibits".

Yet they succeeded. Charges were laid despite there being "not a shred of evidence" other than what Clarke obscurely called "clear intelligence". If the police could produce a charge against Barot in 14 days, and now have 28, to claim that they need yet another 14 strains all credibility. It looks suspiciously like a grab for arbitrary power.

Detention without charge cannot be regarded as simply a matter of police convenience in a good cause. An increasing number of cases are coming to appeal of people wrongly arrested and held in Belmarsh for months or years on such flimsy evidence as CIA say-so (as with an innocent Algerian airline pilot in 2001) or the presence on a computer of "terrorist" material (as with Yassin Nassari in 2007). The same sort of evidence resulted in the Pakistani president's advance entourage being held for 24 hours in January on suspicion of plotting terrorism. When Clarke says the "checks and balances in our system prevent arbitrary detention" he is talking rubbish.

The new bill greatly extends the concept of a criminal offence into the realm of free speech and dissemination, and thus heightens the risk of casual imprisonment. When reading Mein Kampf or wearing an alarmist T-shirt becomes a crime, social tolerance has given way to state supremacy - and taken another chip off civilisation.

If British liberties are to be curbed in the secrecy of a home secretary's office, investigators should indeed spend long nights sleeping under their desks. Clarke admitted that he could not think of any "terrorists" who have been let go for want of 42 days, yet he demands more police power anyway, rather than seek it later "in panic in an emergency".

The police have enough emergency power already. As for panic, that has nothing to do with national security and everything to do with the position of the prime minister. If Gordon Brown needs to appear macho, he can find thugs to arrest, drink laws to change, taxes to raise and citizens to frighten with late-night calls. He has failed to make a case for detaining people for 42 days without charge. MPs should never pass this sinister bill.

Simon Jenkins