Once again Gordon Brown has sought to explain what it is that has driven him to push on with his proposals to extend the maximum period for pre-charge detention, from 28 days to 42 days. “I will stick to the principles I have set out,” he wrote this week in The Times, “and do the right thing.” What are those principles, and what is the right thing?

The context in which this issue comes up raises points on which few will disagree. The threat of terror is real. The availability of new technologies might make it more difficult to apprehend those who seek to cause harm. Globalisation means that networks of individuals extend across national boundaries. No doubt these factors do combine to make more complex the work of those who seek to prevent attacks, but has he satisfied us on the crucial question: does the Prime Minister have evidence that the existing limit on pre-charge detention is somehow inadequate?

The most striking feature of his claim to be acting on principle is his failure to set out any compelling evidence or facts that would justify an extension. The most basic is surely the need for the most important political decisions to be justified by evidence, rather than hypothesis or instinct or expediency. Curiously, Mr Brown makes no

claim that experience since the time limit was raised to 28 days demonstrates that a further extension is needed. Vague allegations are bandied around, mention is made of 200 networks and 30 cells, but Tony Blair's legacy is that the public has long lost confidence in such claims.

His successor has not pointed to a single case in which the 28-day limit was shown to be inadequate, or why an additional 14 days would have addressed the problem. He identifies not one example of any person who has had to be released because the 28-day limit was reached before investigations could be completed. He fails to mention the number of cases in which individuals were charged on or shortly before the limit was reached (I am told he would be hard pressed to do so as the number is very small: well short of a dozen).

On his own account, the Prime Minister abandons the one principle that matters: decent evidence. What seems to have happened is that early on in his premiership Mr Brown took a punt on a number - an arbitrary 42 days - and is now stuck with it. The policy was fixed on the basis of an ill-conceived political objective - tough on terror - and not on the basis of the evidence or any proper consultation. Consequently, the principles now invoked by the Prime Minister seem almost absurd.

“There should always be a maximum limit.” Terrifically reassuring. “The Home Secretary must take [the] decision to Parliament.” If Parliament isn't sitting, are MPs expected to be recalled to vote on the question of whether to extend someone's detention for two weeks? If the event of circumstances to justify that extreme possibility, then why not just invoke the 2004 Civil Contingency Act, which already allows for detention beyond the existing limit?

It is difficult to avoid the conclusion that the manoeuvring and concessions are designed to find some way - any way - to keep the magical number of 42. Saving face, or avoiding another U-turn, is no way to make law on so vital an issue.

The 42-day debacle would be much less worrisome if it stood in splendid isolation. Let him have his 42 days, some might say, and then we can move on. Unfortunately, the Prime Minister's muddled and dangerous thinking on this issue is part of a wider pattern. Mr Blair launched an unremitting assault on the rule of law. He caused Britain to be the only one of the members of the Council of Europe to derogate from the European Convention on Human Rights to allow indefinite detention without charge or trial of certain non-nationals. The courts struck that down.

In legal proceedings in England he wanted to be able to rely on evidence that may have been obtained by torture in legal proceedings in England. The courts struck that down too. After the July 7 bombings Mr Blair's Home Secretary boldly stated that the Government “would not be constrained by international conventions or by the way the judiciary interpreted them”.

Many hoped that those days of Labour's broken relationship with the rule of law and evidence-based decision-making had ended with the arrival of a new prime minister. Sadly, they have not.

Forty-two days should be rejected for what it is, and for the kind of thinking that lurks beneath. The Prime Minister is right to raise issues of principle. However, his governing principle - political survival - should not save this measure, or those that may follow. They are not “the right thing”.

Philippe Sands QC is a practising barrister and Professor of Law at University College London. His new book is Torture Team: Cruelty, Deception and the Compromise of Law

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