"Why are the liberals always on the other side?" asks the fictional French military commander Colonel Mathieu when he is challenged, in The Battle for Algiers, for using torture to fight terror. The film suggests that torture works as a tool of immediate necessity, even if the consequences are a blurring of morality and so final defeat. Four decades on, Mathieu's charge against liberal scruples is still being raised, implicit in the defence of the means being used in a modern battle against Islamic terror. Old conventions and legal obligations are being portrayed as obstacles to victory in a conflict, it is said, whose scope and severity are being recklessly misunderstood. Without supporting torture, the prime minister crystalised this thinking when he asserted last year that"the rules of the game have changed". John Reid's urgent demeanour has done it again in the past week.

Counter-terrorism and justice do not always march in step and nor is the easy response, that justice must always come first, enough of an answer. The dilemmas are more acute. The arrest of 24 suspects in connection with an alleged plot to destroy airliners over the Atlantic may have been a triumph of intelligence and policing that saved many lives. No government could be criticised for acting when it did, on the information it claims to have had. Nor have legal safeguards been broken here. Yet safeguards in other countries are less rigorous. At what point do actions abroad pollute British justice, even if in the short-term they may protect British security?

Reports from Pakistan suggest that much of the intelligence that led to the raids came from that country and that some of it may have been obtained in ways entirely unacceptable here. In particular Rashid Rauf, a British citizen said to be a prime source of information leading to last week's arrests, has been held without access to full consular or legal assistance. Disturbing reports in Pakistani papers that he had "broken" under interrogation have been echoed by local human rights bodies. The Guardian has quoted one, Asma Jehangir, of the Human Rights Commission of Pakistan, who has no doubt about the meaning of broken. "I don't deduce, I know - torture," she said. "There is simply no doubt about that, no doubt at all." If this is shown to be the case, the prospect of securing convictions in this country on his evidence will be complicated. In 2004 the Court of Appeal ruled - feebly - that evidence obtained using torture would be admissable as long as Britain had not "procured or connived" at it. The law lords rightly dismissed this in December last year, though they disagreed about whether the bar should be the simple "risk" or "probability" of torture.

But none of this stops governments acquiescing in torture to acquire information, rather than secure convictions, as British as well as American practice has shown. It has been outsourced to less squeamish countries and denied through redefinition: but it is still torture and still illegal. The former British ambassador to Uzbekistan has provided disturbing evidence of the uneasy boundary between benefiting from torture and encouraging it; so did the Council of Europe's report on rendition in June. The defence, to the extent that anything other than evasion has been offered, is no better than the one provided by Colonel Mathieu in Algiers: it works. But does it? Torture and other illegality can offer authorities a short-term seduction, perhaps even temporary successes. Information provided by torture may have helped foil the alleged airliners plot. But evidence provided uder torture is often unreliable, sometimes disastrously so - and its use always pollutes the broader credentials of torturers and their allies. This battle must be won within the law. Anything else is not just a form of defeat but will in the end fuel the flames of the terror it aims to overcome.