The unanimous ruling by the House of Lords that secret evidence obtained by torture cannot be used against terrorist suspects in British courts is thought-provoking. It suggests that courts already accept “confessions” obtained by the use of electrodes or that the Government is pushing for a relaxation of the rules so that terrorists can be convicted more easily. Courts have always rejected evidence obtained not only by torture but by any form of duress. Indeed, one of the constitutional guarantees that underpins democracy in this country is the near-certainty that any coercive interrogation or strong-arm police tactics will make the evidence obtained inadmissible.

What is strange is the apparent need now for this basic principle of justice to be reaffirmed. So fraught has been the attempt to find a legal response to terrorism, so urgent the need to reinforce national security and so ambivalent the language of some politicians that a perception has arisen that somehow torture has become “acceptable” in the fight against terrorism. It has not.

It is some 30 years since Britain was condemned by the European Court of Justice for the use of hooding, sleep deprivation and stress in the interrogation of IRA suspects. Therefore, British courts have long had not only moral but also international backing for the prohibition on the use of evidence obtained under duress. It makes no difference whether this is exerted at home or abroad: in the unlikely event of a detainee from Guantanamo being prosecuted in Britain, a judge could not admit any confession given to US interrogators, even if they presented it in person and if the court accepted that no detainees had been tortured.

Yesterday’s ruling applies more narrowly to the appeals to the Special Immigration Appeals Commission made by those foreign suspects held without charge in indefinite detention. Some of the evidence against them has been passed on from security officials in their home countries — and in places such as in Algeria or Egypt there is a strong suspicion that torture is, or was, used. The Home Office insisted yesterday that the ruling made little difference, as most of the evidence justifying their detention has been gathered by British Intelligence. Nevertheless, some important consequences follow. First, the burden of proof has been firmly placed on the Government to show that evidence from overseas was not obtained by torture — an almost impossible task, which therefore severely restricts the use of all such information. Secondly, the new undertakings by foreign governments that any suspect returned home will not face torture may now be challeged in court on the ground that the country in question has earlier been accused of using torture. And thirdly, the ruling overturns any notion that torture may be tacitly condoned in certain circumstances in a counter-terrorism policy.

The effect may be to make it harder to put on trial the men in Belmarsh prison. It may also inhibit some foreign intelligence services from passing on information. This could prove awkward if it reduces the flow of tip-offs to forestall terrorist acts. The ruling increases the pressure on British police to find alternative corroborating evidence, which is all to the good. The judgment also underlines a vital principle of evidence admissible in court. But the judges must recognise that future rulings on these vexed subjects are not a test of their own personal liberalism but a challenge for all in our society.