British rights groups and lawyers withdrew Thursday from an inquiry into alleged secret service involvement in the torture of terror suspects, criticising its lack of "credibility or transparency."
Amnesty International, Human Rights Watch and legal charity Reprieve were among 10 groups that said they would not submit any more evidence or attend further meetings with the team organising the inquiry, which is yet to start.
Prime Minister David Cameron announced the inquiry last year to address allegations that secret services were complicit in the torture of violent extremists on foreign soil after the September 11, 2001 attacks on the United States.
But in a letter to the inquiry, the right groups said they would not participate, criticising the terms of reference published last month.
Under the terms, the final decision on whether to put material uncovered by the inquiry into the public domain will rest with the government. Additionally, former detainees and their lawyers will not be able to question intelligence officials.
"We are particularly disappointed that the issue of what material may be disclosed to the public will not be determined independently of government," said the letter.
"Further, that there will be no meaningful participation of the former and current detainees and other interested third parties."
It added: "Our strong view... is that the process currently proposed does not have the credibility or transparency to achieve this."
The inquiry, led by retired judge Peter Gibson, is due to start after the completion of police investigations into alleged complicity by British agents in torture by agents from other countries.
In a second letter, solicitors who represent former Guantanamo Bay detainees also said they would not participate in the inquiry.
"We consider it impossible to advise those whom we represent that the structure and protocols now confirmed for the Gibson inquiry can achieve what are essential ingredients for a public inquiry into grave state crimes," it said.
The letter was signed by leading British human rights lawyers, including Gareth Peirce and Imran Khan.
A statement from the inquiry said it regretted the decision but insisted: "The inquiry will go ahead."
The issue was thrust into the spotlight in February last year when a British court ordered the publication of previously classified details about US interrogators' treatment of British resident Binyam Mohamed.
Ethiopian-born Mohamed was detained in Pakistan in 2002 and says he was interrogated by an officer from British security service MI5 whose role was to support US interrogators.
He was later sent to Guantanamo Bay.
Britain insists it does not participate in, encourage or support the use of torture but several cases have been brought against the government.