The inquiry into whether the UK was involved in alleged torture must be open or it would "only serve to cover up abuses," a UN expert has said.
Juan Mendez said he had heard of "limitations" on the Detainee Inquiry that might "frustrate" it.
Some human rights groups said the inquiry lacked credibility after the government said it would decide which material would be made public.
An inquiry spokeswoman said it would be thorough, fair and transparent.
The inquiry's remit is to establish the extent of the UK government's awareness of, or involvement in, the alleged mistreatment of detainees held by other countries.
Mr Mendez, the UN's special rapporteur on torture, said the UK was doing "absolutely the right thing" in having an inquiry.
But he spoke of "limitations that may frustrate the very object" of the inquiry and said he wanted to learn more about the "parameters that have been set" for it.
"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 public inquiry," he said
"A less than open and transparent inquiry would only serve to cover up abuses and encourage recurrence," he added.
Prime Minister David Cameron announced the inquiry in July 2010 after claims that former Guantanamo Bay detainee Binyam Mohamed was tortured with the knowledge of the UK security services while held by the CIA in Pakistan.
Since then other former detainees have brought legal action against the UK government, claiming they faced similar mistreatment with the knowledge of MI5 or MI6.
This summer it was announced that the final decision on whether material from the inquiry could be made public would lie with the government.
An inquiry spokeswoman said: "The Detainee Inquiry will conduct a thorough, fair and transparent inquiry, as we were asked to do."
She said it was hoped that all evidence would be heard in public but some evidence may have to be given in private if it could, for example, "damage national security or other vital national interests".
She added: "The Detainee Inquiry regrets the decision by some of the detainees and their solicitors not to participate in the inquiry. We hope they will reconsider. But their decision does not affect the continuation of the inquiry.
"We are surprised that Mr Mendez did not speak to the inquiry before he made his public statement."
Amnesty International, Liberty, and Reprieve said the inquiry lacked credibility and could be a "waste of time and public money".
Director of Liberty Shami Chakrabarti said it was time for the government to think again.
Mrs Chakrabarti said: "The UK has shown itself capable of setting up properly independent judicial inquiries into other scandals. Is torture any less serious?"