The UK's human rights watchdog has joined calls for an independent inquiry of claims that the security services were complicit in the torture of more than 20 British terror suspects.
Equalities and Human Rights Commission (EHRC) chairman Trevor Phillips told ministers their vehement denials of the claims had been insufficient to reassure the watchdog or the public.
In a letter to Justice Secretary Jack Straw, he said the Government "needs urgently to put in place a review process to assess the truth or otherwise of all these allegations".
They include claims from Binyam Mohamed, the former Guantanamo Bay detainee, who says he was tortured in Pakistan while held by the CIA, with the knowledge of the British.
Last week his "cruel, inhuman and degrading" treatment by US authorities was revealed after Foreign Secretary David Miliband lost a bid to block the disclosure.
The EHRC has stepped in after a series of reports - including one by the United Nations - detailed alleged cases of collusion as well as the release of the court evidence in the Mohamed case.
In his letter, copied to Foreign Secretary David Miliband and Home Secretary Alan Johnson, Mr Phillips said it would be "inexplicable" for the EHRC to ignore the mounting allegations.
"The Government has stated unequivocally that the allegations are unsubstantiated and that it does not condone or support torture carried out by foreign agencies," he wrote.
"However, the commission does not believe that the Government's response to these allegations is sufficient in itself and believes that not enough has been done to reassure the commission and the public that these allegations are unfounded.
"In the opinion of the commission, the UK government needs urgently to put in place a review process to assess the truth or otherwise of all these allegations."
Any inquiry, he said, should be independently appointed, given "complete access" to relevant materials and held in public wherever that would not pose a "real and substantial" risk to national security.
Another allegation of British complicity in torture comes from last British resident in Guantanamo Bay, Shaker Aamer, who accused British intelligence officers of being present and doing nothing to help him when he suffered torture in US custody at Bagram airport in Afghanistan.
Yesterday the High Court was told that his claims are being investigated by the Metropolitan Police.
Other cases include those of former Guantanamo Bay detainee Moazzam Begg, who alleges British intelligence agents were present when he was abused in Afghanistan by US security services.
Two influential Commons committees have backed an inquiry after raising serious questions about the UK's involvement and suggesting the Government could be in breach of international obligations.
But ministers have been strident in their defence of MI5, dismissing the UN report as "unsubstantiated and irresponsible" and rejecting the need for an outside review.
Mr Phillips also pointed to a recent report by the Human Rights Watch (HRW) organisation which said a five-year study had found "credible" evidence that UK intelligence was aware of abuse.