Joan Smith: Pulling out my toenails doesn't persuade me to tell the truth

When seven Law Lords ruled last week that the Government needs to show that evidence obtained under torture has not been used in cases against foreign terror suspects in this country, there were mutterings that the nation's security had been put at risk. In fact, there's nothing in the ruling to prevent the authorities acting on a tip-off from a foreign intelligence agency about an imminent attack. That is, of course, the scenario most frequently conjured up by people who try to justify beatings, sleep deprivation and electrodes fitted to ticking-bomb terror suspects who won't talk, citing 30 minutes to doomsday in Paris or London or New York.

In real life, it almost never works like that. When Craig Murray arrived as British ambassador in Tashkent, the CIA passed on "intelligence" it had from the Uzbek government - a regime notorious for its use of torture - about a terrorist training camp in the hills above Samarkand. Murray was sceptical, had a look round the area and became convinced the camp did not exist.

He also recalls a terror trial at which an elderly Uzbek retracted a confession identifying two of his nephews as members of al-Qa'ida; he had also claimed, under interrogation, that they visited Afghanistan to meet Osama bin Laden. In court, the man broke down and said he'd never even heard of Bin Laden before, but was willing to say anything after his relatives were tortured in front of him.

By coincidence, another glimpse into the murky world of torture and lies emerged in the US on Friday. The New York Times said the Bush administration based a key pre-war assertion about links between Saddam Hussein and al-Qa'ida on statements made by a Libyan prisoner it had secretly handed to Egypt for interrogation. The "evidence" of Ibn al-Shaykh al-Libi lay behind President George Bush's claim in October 2002 that "we've learnt Iraq has trained al-Qa'ida members in bomb-making and poisons and gases". We were kept in the dark about the identity of the source and the fact that he was an early example of the practice now known as extraordinary rendition.

But Mr al-Libi did not make detailed claims about Iraq until after he had been flown from Afghanistan, where he had been interrogated by the CIA, to Egyptian custody. The agency withdrew some of the intelligence based on his claims in March 2004 after Mr al-Libi said he fabricated them to escape harsh treatment, punching another hole in the credibility of the Bush case for going to war.

Torture is wrong in principle. If we don't defend our values as well as our bodies, we are engaged in little more than tribal warfare. But the practical case against torture is just as strong; I've never understood people who assume suspects tell the truth, rather than whatever they think their captors want to hear, when they're having their toenails pulled out. We should be questioning not the Law Lords' decision but why any Western government would consider accepting dodgy material from unpleasant regimes, given how unreliable it's likely to be.

Nearly every recent terrorist outrage, from 9/11 to Casablanca, from Bali to Madrid, came out of the blue. Only three weeks before the 7/7 attacks, British intelligence chiefs judged that there was no single group "with the current intent and the capability to attack the UK".

Desperate situations encourage a resort to desperate measures, but there's still no moral or practical case for allowing foreign regimes to do our dirty business in their torture chambers.