Big politics is high drama. The moment the 10p deal on tax was done, the cries of “U-turn” and “weakened premiership” were inevitable. So, too, were warnings of carnage to come in a second act of rebellion subtitled “the battle of 42 days”.

I am not qualified to evaluate the extent to which people on low incomes will be adequately protected by a complex system of allowances and credits. What I do know is that 42 days’ precharge detention cannot be “offset”. You can’t be taken from your bed in a dawn raid and held without any sense of the allegation against you with the assurance that your life will be “offset” and “backdated” six weeks later.

Nor can we calculate, let alone compensate, the deficit to national security and unity that institutional injustice creates – how much intelligence dries up; how much harder the task of recruiting police and security officers from minority groups becomes; how much easier it is to radicalise another young mind when the gap between the rhetoric and reality of “British values” widens again.

I also know that whatever the government’s strategy in picking at the scab of Tony Blair’s 90-day defeat, opposition to this policy is not inspired by a whiff of Gordon Brown’s blood. Despite cynicism about politics, the campaign to protect the ancient right to be charged or released has produced true parliamentary heroes. Conservatives, Liberal Democrats and dozens of Labour MPs (many retaining staunch personal loyalty to the prime minister) have united behind high principle and sound practice. What’s more, this alliance was born when the share price of Brown’s government was riding high.

Last summer our new prime minister impressed with his statesmanship. Terror plots, livestock epidemics and serious floods: the horsemen of the apocalypse were seen off with a calm and unifying manner that smacked of honest good governance rather than partisan politics.

To be fair, Brown had long warned of his wish to revisit antiterrorism policy and reconsider the maximum period for which a suspect could be held without charge. However, talk of a new “consensus” and winning “hearts and minds” so contrasted with the divisive posturing of his predecessor that we were optimistic.

It’s no secret that I met the prime minister several times and these conversations, combined with parallel discussions with senior Conservatives and Lib Dems, led some to believe in a golden opportunity. What if political heat could be turned down on terrorism policy? What if the authoritarian arms race of Michael Howard and Blair were to slow down? What if we could be more honest about risk and build public protection upon fundamental rights, freedoms and the rule of law?

The government could discard the self-destructive politics of the “war on terror” and continue the statesmanship of the summer. Opposition politicians could avoid the jibe of being “soft on terror”. Trust in politics as a whole would benefit from the sight of elected representatives uniting on a matter of importance. The government admitted that no case had required more than 28 days and began considering a range of more proportionate alternative measures supported by Liberty and opposition parties.

In return, it was recognised that in a genuine public emergency (described by a Home Office minister as “three 9/11s on one day”), extended detention might temporarily be sanctioned by parliament using existing or refined contingency legislation.

Then the talking stopped. Some time between Donorgate and Northern Rock last autumn, the Home Office published the almost friendless policy now to be found in the fifth counterterrorism bill since 2000.

When you start a movement towards genuine consensus, that movement may continue even after you give up. The government’s own Privy Council review of the use of intercepted telephone calls as evidence found that a “robust legal model” for doing so “would be possible”. Last month 70% of those surveyed in a Liberty/YouGov poll said allowing suspects to be questioned about related offences after they were charged was preferable to an extension of the 28-day limit.

The same poll showed 54% of people thought the motive for the 42-day policy was “to look tough on terror”. The director of public prosecutions (DPP) is working happily and successfully within the current limits. Even the Metropolitan police commissioner admits there is no need for an extension, just a “pragmatic assumption” that such a need will arise in the future.

It is hardly surprising that Liberty’s “charge or release” campaign has drawn support from voices as diverse as a former chief constable and inspector of constabulary, the church’s General Synod and all principal candidates in London’s mayoral race. Unusually, statements of support are coming in from all over the world, from human rights campaigners in Africa, Burma, the United States and even Iran. If Britain crosses the one-month Rubicon, they fear what other societies might do.

Then there is the small matter of the constitutional nonsense that the bill attempts to create. The political buck for triggering the so-called “reserve power” is passed to the DPP and a chief constable who reasonably believe extended detention is necessary for investigating “one or more persons”. The home secretary must agree, but it would be hard for a home secretary not to do so.

Then the scheme becomes really strange. The minister may activate the power because of one case, but thereby changes the law of the land for all terrorism suspects. Subsequent parliamentary debates and votes (no matter how soon) must therefore act as mere ceremonial or risk prejudicing the individual trials that prompted the need for extension. There is no judicial review of the home secretary’s decision to activate the power. The overall picture is of a power dressed up as “exceptional” that may be used in any individual case where police and prosecutors would like more time. That is no basis for law-making in the oldest unbroken democracy on Earth.

Unless the government returns to consensus, more parliamentary theatre is unavoidable. Minor fig-leaf concessions will not buy off MPs. They know the details of this argument almost as well as the sizes of their majorities. Government rain may be opposition sunshine, but not when it comes to national security. Governments can also change the weather and occasionally everyone wins.

Shami Chakrabarti is the director of Liberty.

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