The declassification of the executive summary of the Senate Intelligence Committee's torture report in December was supposed to lay bare the extent of UK cooperation in America's post-9/11 rendition program.

This heavily redacted document gave no clues about British complicity in torture, however. Now the UK is under further pressure to reveal the nature of any redactions it requested from the US, following an explosive public admission by a senior Bush administration official that a CIA "black site" operated on the British territory of Diego Garcia.

Colin Powell's former chief of staff Lawrence Wilkerson revealed to VICE News on Friday that interrogations did take place on the remote atoll. "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," Wilkerson added.

It has already been established that the UK lobbied the US over the contents of the report. Former foreign secretary William Hague admitted in July 2014 that the UK government "made representations" to the US over the report's contents.

"We have made representations to seek assurances that ordinary procedures for clearance of UK material will be followed in the event that UK material provide[d] to the Senate committee were to be disclosed," Hague explained in a letter to human rights group Reprieve.

Weeks later, logs released under the Freedom of Information Act showed that the UK ambassador to the US met members of the Senate Intelligence Committee 11 times between 2012 and 2014 — while they were investigating the CIA's rendition program and deciding how much of the report to make public.

It has now emerged that the UK Foreign Office refused to answer parliamentary questions about how the UK lobbied the US over redactions to the Senate Intelligence Committee's torture report, fueling concern that it is attempting to conceal its involvement in the CIA's post-9/11 program.

Caroline Lucas, the Green Party MP for Brighton Pavilion, asked the Foreign Secretary on how many occasions UK intelligence agencies discussed the report with US officials and whether the Foreign Secretary was consulted over which redactions to request from the report.

"It is the longstanding policy of successive governments not to comment on intelligence matters," Foreign Office minister Tobias Ellwood replied twice to Lucas on January 28.

It also emerged over the weekend that, prior to Wilkerson's disclosures to VICE News, UK Prime Minister David Cameron hinted that Britain's alleged role in torture could be the subject of a new investigation by an independent inquiry.

"Once the ISC [Intelligence and Security Committee] 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 said in a letter to Reprieve.

Cameron's comments are significant because the UK ISC is currently investigating whether British officials were complicit in torture overseas — but critics claim that the committee is not independent enough from Whitehall for its inquiry to be credible. Now Reprieve says the prime minister's comments suggest that even he does not believe the ISC will get to the bottom of British involvement in torture.

The UK's official rendition inquiries have experienced a number of setbacks over the years. The ISC investigation follows an earlier independent investigation, led by retired judge Sir Peter Gibson, which had to be scrapped in 2012 after human rights groups boycotted it and London's Metropolitan police began inquiries into Britain's role in the rendition of two detainees, Abdel Hakim Belhaj and Sami al-Saadi, who were allegedly tortured in Libya.

Gibson's interim report into the pair's rendition found no evidence of UK participation in torture. It did, however, raise 27 questions about potential British involvement in the CIA's global rendition program.

Confirmation that the British were complicit in the rendition and torture of detainees could leave the government vulnerable to legal action. The European Court of Human Rights ruled last year that detainees had been held and tortured in a secret CIA prison in Poland.

Donald Campbell, an investigator for Reprieve, told VICE News: "The Government's refusal to come clean about the part Britain played in the CIA's torture program is shameful — especially in light of last week's reports that Diego Garcia was used for interrogations.

"Ministers have admitted that UK agencies demanded redactions in the Senate's torture report — so why won't they say whether they signed off on them?

"If our elected representatives don't know what was kept out of the Senate report, how can they be so sure that it was done for legitimate reasons, and not simply to avoid embarrassment about UK complicity?"