David Cameron has raised the prospect that Britain’s alleged role in torture could be the subject of a second investigation by an independent inquiry.

The move, which would be applauded by human rights groups, could see the intelligence and security services forced to give evidence before a judge, which may present an unwelcome distraction for MI5 and MI6 as they seek to combat the growing threat from radical Islamist groups.

The Intelligence Security Committee (ISC) is investigating whether British officials were complicit in torture overseas, but critics claim that the committee is too close to the Whitehall establishment to conduct a thorough and credible inquiry. An earlier independent investigation, led by a retired judge, Sir Peter Gibson, was scrapped when the Metropolitan police launched inquiries into Britain’s role in the rendition of two detainees to Libya, where they were allegedly tortured.

Abdel Hakim Belhaj and Sami al-Saadi, opponents of Colonel Gaddafi’s regime, claim they were interrogated by British intelligence officers who were aware they were being tortured by their captors. The CPS is deciding whether there is a case for prosecution.

In an interim report, Gibson said he had not found any evidence of UK agents participating in torture, but he raised 27 serious questions about their potential involvement in the CIA’s rendition programme, which involved interrogating terrorist suspects at “black sites” around the world. Now, in a letter to the human rights group Reprieve, Cameron says an independent inquiry may have to be revived. “Once the ISC has reported and the outcomes of the police investigations are clear, we will be able to take a final view on whether another inquiry is necessary to add any further information of value to future policy-making in this area,” he writes.

His comments have been seized on by critics of the ISC. “This suggests that even the prime minister thinks the ISC won’t get to the bottom of British involvement in torture,” said Reprieve’s executive director, Clare Algar. “So why doesn’t David Cameron just stick to his original promise of an independent, judge-led inquiry?

“The British public deserve to know what part the UK played in the CIA’s shameful torture programme, but the ISC lacks the powers and the independence needed to be sure of getting to the truth.”

Cameron’s comments were made before a former senior US government official told Vice News that he had learned that the British overseas territory of Diego Garcia had been used to interrogate terrorist suspects. Human rights groups fear that the atoll in the Indian ocean was used to facilitate the CIA’s torture programme and have long suspected British collusion.

Lawrence Wilkerson, who was chief of staff to the US secretary of state Colin Powell from 2002 to 2005, said his CIA contacts “indicated … that interrogations took place” on Diego Garcia as part of the CIA’s rendition and torture programme.

“You might have a case where you simply go in and use a facility at Diego Garcia for a month or two weeks or whatever and you do your nefarious activities there,” he told Vice.

In 2007, the ISC produced a report which said there was “no evidence that the UK agencies were complicit in any ‘Extraordinary Rendition’ operations”.

However, Wilkerson appears to be sceptical of such views. “It’s difficult for me to think that we could do anything there of any duration to speak of without the British knowing – at least the British on the island – knowing what we were doing,” he said.

His comments contradict years of assurances by British ministers that “no US detainees have ever been held on Diego Garcia”.

Pressure has been growing on the government to announce an independent inquiry following the publication of a US Senate report confirming that the CIA used torture after 9/11. Critics claim that the ISC is compromised because it is largely made up of establishment figures whose meetings are mainly conducted in secret. It cannot compel witnesses to give evidence and those who do are not questioned under oath.

In December a coalition of human rights NGOs, including Reprieve, Amnesty International, Justice and Liberty, wrote to Cameron urging him to “establish an inquiry into UK involvement in rendition and torture, with the powers and independence it needs to get to the truth, and a mechanism to allow the meaningful participation of victims”.

Cameron acknowledged the need for such an inquiry when he appointed Gibson in 2010. “I do not think for a moment that we should believe that the ISC should be doing this piece of work,” he said. “For public confidence – and for independence from parliament, party and government – it is right to have a judge-led inquiry.”