An inquiry into allegations of British complicity in torture will "only serve to cover up abuses" if it is not fully open and transparent, a UN expert has warned.
Juan Mendez, the UN's special rapporteur on torture, said he was concerned about "limitations" on Sir Peter Gibson's inquiry "that may frustrate the very object of such an exercise".
Human rights groups and lawyers have said they will not take part in the inquiry following the announcement that the final decision on whether material can be made public will rest with the Government.
Mr Mendez said: "I've seen from my work around the world that the way to deal with the cancer of torture is to fully root it out with a wide-ranging, independent and fully
"A less than open and transparent inquiry would only serve to cover up abuses and encourage recurrence."
Prime Minister David Cameron announced the inquiry in July last year after claims that former Guantanamo Bay detainee Binyam Mohamed was tortured with the knowledge of the British security services while held by the CIA in Pakistan.
A number of other former detainees have since brought legal action against the UK Government, claiming they were subjected to similar mistreatment with the knowledge of MI5 or MI6.
Allegations have also been made of UK involvement in the extra-judicial transfer, or rendition, of terror suspects between countries since the 9/11 attacks on the US.
Lawyers representing form Guantanamo Bay detainees were among those who said they were pulling out but a an inquiry spokesperson insisted the process would go ahead.
"The Detainee Inquiry will conduct a thorough, fair and transparent inquiry, as we were asked to do," the spokesperson said.
"We wish to hear all evidence in public unless the panel decide it should be heard in private.
"A decision that evidence should be given in private may be due to a number of reasons including: whether the evidence would, if revealed in public, damage national security or other vital national interests, and any other reasons such as health or security that would make it difficult for the witness to appear or to be entirely frank in public."