The attorney general was under intense pressure tonight to order a wider series of police investigations into British complicity in torture after one of the world's leading human rights organisations said there was clear evidence of the UK government's involvement in the torture of its own citizens.

After an investigation spanning more than a year, Human Rights Watch (HRW) today condemned Britain's role in the torture of terror suspects detained in Pakistan as cruel, counter-productive and in clear breach of international law.

Critically, a report published today by HRW – entitled Cruel Britannia: British Complicity in the Torture and Ill-treatment of Terror Suspects – draws upon corroborative evidence received from the Pakistani torturers themselves.

Researchers at the New York-based NGO spoke to Pakistani intelligence agents directly involved in the torture who say their British counterparts knew they were mistreating British terrorism suspects. These agents said British officials were "breathing down their necks for information" while they were torturing a medical student from London, and that British intelligence officers were "grateful" they were "using all means possible" to extract information from a man from Luton being beaten, whipped, deprived of sleep and threatened with an electric drill.

"UK complicity is clear," the report says, adding that it had put the government in a "legally, morally and politically invidious position".

The attorney general, Lady Scotland, has already asked Scotland Yard to investigate two alleged cases of British complicity in torture, one involving Binyam Mohamed, a British resident tortured in Pakistan and Morocco, and a second involving an unnamed MI6 officer and an alleged victim not identified.

William Hague, the shadow foreign secretary, said it was vital that Scotland be asked to examine all cases where there is credible evidence of British complicity. "We believe that any credible allegations of UK complicity in torture should be referred to the attorney general to establish whether police investigation is necessary," he said.

"The prime minister made a commitment to do just that. It is up to the government now to say what it will do in light of the allegations contained in the report."

The former shadow home secretary David Davis said the report was "astonishing", in that it "destroys the last remnants of any defence the government might have". He called on the government to hold an independent judicial inquiry.

HRW added to the growing number of calls for an inquiry into Britain's role in the torture. Among those issuing demands are parliament's joint committee on human rights, the Liberal Democrats, Amnesty International, and the former director of public prosecutions Sir Ken Macdonald. Lord Carlile, the government's independent reviewer of counterterrorism legislation, Lord Guthrie, a former chief of defence staff, and Lord King of Bridgwater, a former Conservative defence and Northern Ireland secretary, have also called for an inquiry.

HRW pointed out todaythat the government may have little choice but to investigate British complicity, not only because a failure to do so is threatening to undermine its core values, but because it is a requirement of international law.

"The convention against torture requires states to reinforce the prohibition against torture through legislative, administrative, judicial and other measures," the report says.

Privately the Conservatives are aware that they may inherit this problem if they win the next election.

Asked todaywhether the government's repeated insistence that it does not condone, encourage or solicit torture was any longer credible, a Foreign Office spokesperson replied with the prepared statement: "There is no truth in the more serious suggestion that it is our policy to collude in, solicit, or even directly participate in abuses of prisoners." Human Rights Watch had not suggested any direct British participation in torture.

The Guardian reported this year that an official government policy, devised to govern British intelligence officers while interrogating people held overseas, resulted in people being tortured, and that Tony Blair, when prime minister, was aware of the existence of this policy.

The Guardian has repeatedly asked Blair about any role he played in approving the policy, whether he knew that it led to people being tortured, whether he personally authorised interrogations that took place in Guantánamo and Afghanistan as well as Pakistan, and whether he made any effort to change the policy. Blair's spokesman responded by saying: "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."

When the Guardian pointed out to Blair that it had not suggested that he had authorised the use of torture – as opposed to asking him whether he had authorised a policy that led to people being tortured – and that his spokesman had not answered the questions that were asked, his spokesman replied: "Tony Blair does not condone torture, has never authorised it nor colluded in it. He continues to think our security services have done and continue to do a crucial and very good job."