Bob Marshall-Andrews: This folly has provoked a unique alliance

The strong and growing opposition to detention for 42 days has created strange and wonderful bedfellows. Lord Peter Goldsmith, the former Attorney General, joins the usual Labour suspects in predicting ethnic and racial tension. (In view of his responsibility for the Iraq war this may reek of atonement, but the apostasy remains remarkable nonetheless.) Lord Falconer, devoid of his many jobs, embraces the Tory and Liberal opposition, while Liberty and the Director of Public Prosecutions drink happily from the same cup.

In the Commons strange enemies emerge. David Davis (Tory and Territorial SAS) leads a charge containing numerous former soldiers against the professional politicians of New Labour. (It is difficult indeed for the Home Secretary to brand opponents "soft on terror" when those opponents were on IRA death lists before she became a parliamentary candidate.)

This extraordinary and unique alliance requires from the Government careful analysis and measured response. It is receiving neither. Instead, two dangerous, possibly deadly, misconceptions are growing among loyal ministers and backbench MPs.

The first is that the opposition to 42 days is based not upon principle but is an opportunistic coincidence of self-interested forces – Blairite, right wing, left wing and liberal – united only in their dislike and loathing of Gordon Brown and their desire (through widely different motivations) to have done with him. In other words it is part of a fashionable feeding frenzy in which many different species of vulture may gorge.

The second, closely associated with the first, is that it simply doesn't matter; that the British electorate, Sun readers or Mondeo drivers all, are indifferent to the withdrawal of civil liberties and freedoms (particularly in the case of those who are, or appear to be, foreign).

These happy fallacies ignore two related facts. Firstly, this near universal rejection of extended detention reflects a cumulative and growing sense of alarm at a persistent erosion of liberty on many fronts, from assaults on jury trial to the creation of a vast, secret identity base. This sense of alarm has itself achieved growing urgency and importance as a result of a collapse of public trust caused mainly, but not entirely, by the duplicity of the Iraq war.

The 42-day issue throws this into the sharpest focus. The Government has offered no evidence to support the necessity for 42 days and is therefore bound to rely upon trust. Implicitly, the Government maintains that dangers and threats exist which cannot be fully explained but which require faith in the process of government and the intelligence services. The dismal echoes of Iraq could not be more resonant.

This position is made even worse by the failure to explain the attempt two years ago to coerce Parliament into a vote for 90 days detention with minimal safeguards. Assertions that this was necessary ("Trust us") have been revealed, self evidently, to be false. Many of my Labour colleagues drawn unwillingly into the whips on 90 days against their instinct and judgement now remain justifiably angry and unconvinced.

So whence 42 days? Or 50? Or 63? Do I hear 80? This is the politics of the auctioneer and barrow boy employed upon civil, peacetime liberties that have endured for eight hundred years.

Even the so-called safeguards rely upon trust. Parliament is to be given a power to review the 42-day extension period in individual cases. This quasi-judicial role is totally unworkable. Parliament has neither the capacity nor the constitution to investigate individual cases.

Finally, when MPs vote today we must confront the most dangerous fallacy based upon a perception of public apathy – that they don't care. We must understand that the British do not articulate liberties easily any more than they define them in lists or guard them as properties or beneficence gratefully received from their masters above. For us, political and personal freedoms are not gifts or indulgences, they are defining characteristics as a nation.

This abiding but subliminal confidence may occasionally appear as indifference, but it is a dangerous illusion. History is littered with political casualties who misread or ignored robust and historical link between the British and their liberties. It represents, insofar as anything can, the authentic nature of British patriotism. British people have awoken to the major threat posed by the terrorist: namely that he will provoke government to change the very nature of their freedoms. It is that growing perception that is reflected in the eclectic voices now joined in extraordinary opposition.

If Labour MPs of principle accede to the whip tonight they will deserve the censure that will inevitably follow led by the strangely united political forces whose opposition this folly has provoked.

The author is the Labour MP for Medway.