A British resident who says he was tortured before being sent to Guantánamo Bay said yesterday he may give evidence on behalf of an MI5 officer to ensure that senior figures within the government are held to account for any involvement in his treatment.

Binyam Mohamed spoke to the Guardian after the attorney general called in the Metropolitan police to investigate claims that MI5 had colluded in his interrogation.

Mohamed said he was determined that the officer, known only as Officer B, should not be scapegoated. "It's very important that we get to the truth, for everyone in the future," he said.

The attorney general, Lady Scotland, announced the unprecedented move in light of damning evidence that Britain's security and intelligence agencies colluded with the CIA in Mohamed's inhuman treatment and secret rendition.

She said the police inquiry would look into "possible criminal wrongdoing" in what the high court described as Mohamed's unlawful questioning.

After being arrested at Karachi airport in April 2002, while travelling on a false passport, Ethiopian-born Mohamed was held incommunicado in Pakistan, Morocco, and Afghanistan, before being flown to Guantánamo Bay in 2004.

He was released last month after the US dropped all charges against him, including claims that he was trying to make a "dirty bomb".

The attorney general's move yesterday was welcomed by MPs, lawyers and human rights groups. However, they made clear that it did not go far enough, and that a judicial inquiry was needed to investigate the full extent of Britain's alleged collusion in torture.

Scotland said that after reviewing a "substantial body of material, much of it highly sensitive", and after consulting the director of public prosecutions, Keir Starmer QC, she concluded that the evidence should be passed to the police.

She said she hoped the investigation would be carried out as "expeditiously as possible, given the seriousness and sensitivity of the issues involved".

The evidence includes open and secret high court hearings where Officer B was questioned about his interrogation of Mohamed when he was being held in Pakistan in 2002. The court heard that MI5 provided the CIA with material to interrogate Mohamed, even though it said it had no idea at the time where he was being held and in what condition he was in.

In a case Lord Justice Thomas described as "deeply disturbing", involving "many and very troublesome issues", the high court concluded: "The conduct of the security service facilitated interviews by or on behalf of the United States when [Mohamed] was being detained by the United States incommunicado and without access to a lawyer." They court added: "Under the law of Pakistan, that detention was unlawful ."

Much of the evidence is still being suppressed following gagging orders demanded by David Miliband, the foreign secretary, and the US authorities. Leading politicians and campaigners yesterday said that the issue went far beyond the particular case of Officer B, and that a broader inquiry was needed.

The Conservative leader, David Cameron, called for a "targeted and clear review ... to get to the bottom of whether Britain was knowingly or unknowingly complicit in torture".

The Liberal Democrat leader, Nick Clegg, said: "It is not enough for Gordon Brown to say the government does not endorse torture. There remain serious questions concerning how far senior political figures were implicated in these alleged practices."

Tayab Ali, a London solicitor who is representing two other men who say they were tortured by Pakistani intelligence agents before being questioned by MI5 officers, said that a judicial inquiry was required because, without one, "there is every danger that this will happen again".

Clare Algar, executive director of the legal charity Reprieve, said: "The really important thing is that this does not become a witch-hunt of the [MI5] agent in question. Any investigation needs to look at the person at the top of the chain of command."

Andrew Tyrie, chairman of the Commons all-party parliamentary group on extraordinary rendition, referred to the high court's description of UK involvement in Mohamed's interrogation as going "far beyond that of a bystander or witness to the alleged wrongdoing", and called for a wider investigation.

The prime minister, Gordon Brown, speaking from Brazil, said that the government would not tolerate or endorse torture.

The US alleged that Mohamed had undergone terrorist training at al-Qaida camps in Afghanistan. Mohamed claimed he visited the country to kick a drugs habit and to study Islam.