Tony Blair was aware of the ­existence of a secret interrogation policy which ­effectively led to British citizens, and others, being ­tortured during ­counter-terrorism investigations, the Guardian can reveal.

The policy, devised in the aftermath of the September 11 attacks, offered ­guidance to MI5 and MI6 officers ­questioning detainees in Afghanistan who they knew were being mistreated by the US military.

British intelligence officers were given written instructions that they could not "be seen to condone" torture and that they must not "engage in any activity yourself that involves inhumane or degrading treatment of prisoners".

But they were also told they were not under any obligation to intervene to prevent detainees from being mistreated.

"Given that they are not within our ­custody or control, the law does not require you to intervene to prevent this," the policy said.

The policy almost certainly breaches international human rights law, according to Philippe Sands QC, one of the world's leading experts in the field, because it takes no account of Britain's obligations to avoid complicity in torture under the UN convention against torture. Despite this, the secret policy went on to underpin British intelligence's ­relationships with a number of foreign intelligence agencies which had become the UK's allies in the "war against terror".

The policy was set out in written instructions sent to MI5 and MI6 officers in January 2002, which told them they might consider complaining to US officials about the mistreatment of detainees "if circumstances allow".

Blair indicated his awareness of the existence of the policy in the middle of 2004, a few weeks after publication of photographs depicting the abuse of detainees at Abu Ghraib prison in Iraq.

It was around this time, David ­Miliband, the foreign secretary, told MPs on ­Tuesday, that the policy was changed, becoming more "comprehensive and formal".

In a letter to the intelligence and ­security committee (ISC), the group of MPs and peers that provides political ­oversight of the UK's security and ­intelligence ­services, on May 24 2004, Blair said that rather than considering making a ­complaint, "UK intelligence personnel interviewing or witnessing the interviews of detainees are instructed to report if they believe detainees are being treated in an inhumane or degrading way".

The Guardian has learned from a ­reliable source that MI5 officers are now instructed that if a detainee tells them that he or she is being tortured they should never return to question that person.

It remains unclear what Blair knew of the policy's consequences. The Guardian has repeatedly asked him what role he played in approving the policy, whether he was aware that it had led to people being tortured, and whether he made any attempt to change it.

His spokesman said: "It is completely untrue that Mr Blair has ever authorised the use of torture. He is opposed to it in all circumstances. Neither has he ever been complicit in the use of torture.

"For the record, also, Mr Blair believes that our security services do a superb job of protecting our country in difficult ­circumstances and that it is not surprising following the attacks of September 11 2001 that there was a heightened sense of the dangers the country faced from terrorism. None of this amounts to condoning the use of torture."

When the Guardian pointed out to Blair that it had not suggested he had authorised the use of torture, but had asked whether he had played any role in the approval of a policy that led to people being tortured, his spokesman replied: "Tony Blair does not condone torture, has never authorised it nor colluded in it at any time." But there is growing evidence of MI5's ­collusion in the torture of British ­terrorism suspects in Pakistan, where officers of the Inter-Services Intelligence ­directorate (ISI), an agency whose routine use of ­torture has been widely documented, were asked by MI5 to detain British ­citizens and put questions to them prior to an ­interrogation by MI5 officers.

Two high court judges say they have seen "powerful evidence" of the torture of Binyam Mohamed, the British ­resident who returned from Guantánamo Bay in February, before he was questioned by an MI5 officer in May 2002.

In a separate case, a court has heard that MI5 and Greater Manchester police drew up a list of questions to be put to another man, Rangzieb Ahmed, who was detained by the ISI in August 2006, despite having reason to believe that he was in danger of being tortured.

By the time Ahmed was deported to the UK after a lengthy period of unlawful detention three of his ­fingernails were missing.

Several other men have come forward to say they were questioned by British intelligence officers after suffering brutal torture at the hands of Pakistani agents, and there have been similar allegations of British collusion in the torture of ­British citizens in Egypt, Bangladesh and the United Arab Emirates.

While a small number of the victims were subsequently tried and convicted in the UK, most were released without charge.

International concern about ­Britain's involvement in torture has been ­mounting for some time. In February Martin Scheinin, a UN special rapporteur on human rights, reported that British intelligence ­personnel had "interviewed detainees who were held incommunicado by the Pakistani ISI in so-called safe houses, where they were being tortured".

Scheinin added that this "can be ­reasonably understood as implicitly condoning torture."

In March, after the Guardian disclosed the existence of the interrogation policy, and reported on the growing number of allegations of British collusion in torture, Gordon Brown announced that the policy was to be rewritten by the ISC.

In what was seen at Westminster as an acknowledgement that the secret policy had been open to abuse, Brown also pledged that the rewritten policy would be made public and that a former appeal court judge would monitor the ­intelligence agencies' compliance with it, and report to the prime minister each year.

On Tuesday Miliband said the existing policy, as amended in 2004, would not be published.

But the discovery that Blair was aware of the secret interrogation policy appears certain to fuel the growing demand for an independent inquiry into aspects of the UK's role in torture and rendition.

So far, those who have called for such an inquiry include the Conservative and Liberal ­Democrat leaders David ­Cameron and Nick Clegg; Ken Macdonald, a former director of public prosecutions; Lord ­Carlile of Berriew, the government's ­independent reviewer of counter-­terrorism ­legislation; Lord Howe, who was foreign secretary between 1983 and 1989 in the Thatcher government; and Lord Guthrie, a former chief of defence staff.