Law and principle are lost in the crazy politics of 42 days

Who would be a Labour MP this week? After brutal whipping and endless spin about "toughing it out" while "offering concessions", it still boils down to this. Is it right, or necessary, or productive to our safety, to detain a suspected person for six weeks without charge - without knowing what they are accused of?

Some view this as a trial of political machismo. Liberty does not. Political parties play a part in democracy, but I respect the conscience, courage and conviction of individual parliamentarians more. I will always regret the crazy politics that "42 days" has become and our inability - despite endless efforts - to persuade the Brown government towards a break with the recent past.

Liberty colleagues have had discussions with Labour MPs who have been thoughtful in their engagement. My confidence is such that I believe on a free vote in the Commons, the 42-day measure would be easily defeated. But at the fag end of the misnamed, misjudged "war on terror", abortion time limits are left to the conscience and detention time limits are not. The margin will be tight.

Some MPs have expressed fears that this issue might become a running sore for their party. As Europe split the Conservatives in the 1990s, so civil liberties might create a fault line through Labour. Government admissions that there is no need for an extension have left many angry about being forced to revisit this issue so soon after the vote in 2005.

In democracies where people are presumed innocent, we arrest on suspicion, charge with evidence and convict after proof. These principles were built on centuries of struggle. Even this tradition risks hundreds or thousands of people being plucked from their beds and detained under terror laws. A smaller number will be charged with something, and some eventually convicted.

Humans - including policemen - are imperfect. Hence the age-old wisdom of prompt charging following arrest, so even the most heinous murders must result in charge within four days. Hence the one-day limit in Canada, two in the US, and periods of a week or less all over the free world. Ministers have been quick to try to rubbish my organisation's extensive research into comparisons but have produced none of their own. The Council of Europe's commissioner for human rights, Thomas Hammarberg, is in no doubt where the UK stands. He says the "British government's suggestion to allow terrorism suspects to be detained for 42 days without charge would be way out of line with equivalent detention limits elsewhere in Europe".

After charge, innocents may still be held for months pending trial for a complex conspiracy, but at least they know why. At least they and their family and lawyers can prepare a defence in the hope of vindication in court. Contrast the nightmare of a thousand hours in custody followed by unceremonious release back into the community. How will that help social cohesion and national unity? Ministers claim to have consulted "senior Muslims". My concern is with the junior ones who become alienated and radicalised.

The security minister Admiral Lord West was both provocative and practical when he said people should "snitch" on those they suspect of involvement in terrorist activity. How much intelligence might be lost in the anxiety that providing information risks an innocent neighbour disappearing for six weeks? I debated this on the radio with a government loyalist. "Six weeks' detention is not so long," he said, "a school holiday". Before I could react, the Irish-born broadcaster cut in: "I was detained under the Prevention of Terrorism Act." It clearly hadn't felt like a holiday to him.

So, on the government's own case, there is no need for this power; but they wheel out selected policemen who say there may be a need one day. They are slow to explore a range of less sexy alternatives to the constant escalation of the detention arms race.

Serving police officers have also told Liberty of their opposition to the government's plans. But most fear recrimination if they go public. The former chief constable of the West Midlands, Lord Dear, is no stranger to terrorist threats. He was personally targeted by the IRA and yet calls extended detention a "propaganda coup for al-Qaida". In his experience the "best course for a terrorist was to provoke a government to overreact to a threat by eroding civil liberties, increasing executive powers and diminishing due process by the denial of justice".

One week from the vote, we are told ministers can have their authoritarian cake and eat it with sugar-free "concessions". The home secretary even says her last-minute amendments transform the 42-day power into a liberal enhancement of existing emergency powers. The joint parliamentary committee on human rights disagrees: "The safeguards in the bill, even after the potential government amendments, are inadequate to protect individuals against the risk of arbitrary detention."

Confidence tricks catch only those unwilling to look beyond the smoke and mirrors. First, the "grave and exceptional terror threat" is broad enough to catch any suspected terror plot anywhere in the world, rather than a genuine emergency in Britain.

Second, the threat is a phrase for a statement to the Commons, not a legal precondition for detention. That means that the power to extend detention is still triggered by individual cases rather than general emergencies. Parliament becomes a farcical star chamber charged with discussing individual cases without prejudicing potential trials.

Finally, there is no judicial review of the decision to turn on the power. The only role left to a judge would be to authorise detention week on week without evidence or charges to examine.

On morning radio Jacqui Smith asked for our trust. Since when was trust in today's home secretary a basis for suspending the rule of law? It is part of her job to plan for horrific scenarios. It is the job of her parliamentary colleagues to consider her proposals in future home secretaries' hands. This is not a vote of confidence in this government, but about confidence in parliament's ability to hold all governments to account.

Who would be a Labour MP next week? Vote against this posture and face the whips in the morning. Or vote for it and face your grandchildren forever.

Shami Chakrabarti, director of Liberty www.libertyhumanrights.org.uk