Sir Richard Dearlove, the former head of the Secret Intelligence Service, MI6, has criticised two current US policies in the "global war on terror" saying they would have been "illegal" under British law.

Sir Richard, formerly known in Whitehall as "C" and now master of Pembroke College, Cambridge, singled out CIA rendition flights and the indefinite detention of prisoners in Guantánamo for rebuke.

Speaking at the Aspen Ideas Festival in Colorado before an audience of global politicians, experts and commentators last week, Sir Richard also said the west was "doomed" unless it "reclaimed the moral high ground".

According to an online report on the website of the Atlantic magazine, the co-sponsor of the conference, Sir Richard was questioned about which policies he was referring to when he said they "would have been illegal under British common law".

Sir Richard replied: the "whole Guantánamo operation" and CIA "rendition", where suspected terror suspects are knowingly transferred to third countries where torture was practised.

When contacted by Guardian Unlimited to confirm his comments Sir Richard said he "wouldn't disassociate" himself from the Atlantic's report and that he was merely expressing a view that was held by many leading UK lawyers.

"Terrorism is an extreme form of political communication," he said. "You want to be sure that, in your response, you don't end up amplifying the messages that terrorists are trying to convey," he told his audience.

The former British spy chief told the conference that it was a "strategic necessity" that the US held to its "best traditions" because it was easier to recruit agents if they believed they were acting in a "good cause".

"We need to think very carefully about long-term strategy. If we don't hold to the moral high ground in the medium to long term it will much more difficult to conduct a successful counter-terrorist strategy.

"The general approach of the "war on terror" made sense for a while after 9/11 but as time passes it may well need adjustment," he told Guardian Unlimited in his first on-the-record comments to a British news organisation since leaving MI6.

He insisted that he wasn't critical of "day-to-day" operational necessities but wanted the wider strategy of the "war-on-terror" to be reconfigured.

While his view of Guantánamo is shared by many in the British government, including the present attorney general, Lord Goldsmith, his comments on CIA rendition in front of an American audience are more controversial.

As MI6 chief, Sir Richard would have maintained a very close relationship with colleagues in the CIA, where considerable amounts of intelligence on the terror threat would have been shared.

His tenure coincided with a period when it is alleged that al-Qaida suspects were being "rendered" to third countries and finally to Guantánamo Bay.

Intelligence analysts believe it is inconceivable that he didn't know about CIA rendition flights even though the first reports of the practice emerged as he retired from MI6 in May 2004.

Under the 1988 Criminal Justice Act, MI6 Officers can be charged with a criminal offence if it is found they have acquiesced in an act of torture even if it takes place outside the UK.

As the head of the Secret Intelligence Service, Sir Richard never gave on-the-record comments and even had his identity obscured when he gave evidence to the Hutton inquiry into the circumstances surrounding the death of the government scientist Dr David Kelly in 2003.

He also recommended in Aspen that when western democracies wanted to infringe civil liberties in the name of counter-terrorism it was better to do this through legislation rather than executive action as this ensured political debate and greater legitimacy.

Sir Richard's comments were echoed by former US secretary of state Colin Powell and Republican senator and presidential hopeful John McCain, who told the conference that it was time to close down Guantánamo.

In the closing keynote session Karl Rove, President Bush's chief political advisor, responded to the demands for closure, saying: "And do what? When we close Gitmo, the question is 'what do we do with the bad guys at Gitmo?' What do you do with them?"

Sir Richard was also asked about the minutes of Downing Street meeting, leaked to a newspaper, in which he told Tony Blair in the summer of 2002 that in the US "intelligence and facts were being fixed around the policy" and that an invasion was "seen as inevitable".

His leaked comments had caused political controversy in the United States.

According to the Atlantic, he responded: "I am less than two years out of government, and I have my pension. Check out the archives in Pembroke College, Cambridge in a hundred years."

But he added: "The version of that memo that is most often quoted was not the final version."

After 33 years in the Secret Intelligence Service, MI6, Sir Richard Dearlove was appointed head of the agency by Robin Cook in 1999 and served as chief until 2004 during one of the agencies most turbulent and controversial periods following 9/11 and the lead-up to the Iraq invasion in 2003.

MI6 became embroiled in the controversy about the quality of its intelligence on Iraq's weapons of mass destruction amid accusations that its qualified assessments were turned into hard facts for political reasons.

As head of the agency, Sir Richard never gave on-the-record interviews to journalists, although he did have hold private briefings including one with Kevin Marsh and John Humphrys of Radio 4's Today programme, weeks before Andrew Gilligan reported that Downing Street had "sexed-up" its September 2002 Iraq dossier.

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