Former senior judge Peter Gibson will lead the probe, which is expected to start up by the end of the year and report back within 12 months. Cameron said the inquiry would investigate whether British intelligence officers were involved in the "improper treatment of detainees held by other countries in counterterrorism operations overseas, or were aware of improper treatment of detainees in operations in which the U.K. was involved."

The prime minister had promised the inquiry during this year's election campaign, saying it was needed to end the uncertainty around the country's role in the alleged torture of British nationals since the Sept. 11, 2001, attacks.

Several former detainees at the U.S. military prison at Guantanamo Bay, Cuba, are suing the British government for damages over its alleged involvement in their treatment and detention. Other detainees have said they were rendered to prisons in places such as Pakistan and Morocco and were subsequently tortured. "The longer these questions remain unanswered, the bigger the stain on our reputation as a country that believes in freedom, fairness and human rights grows," Cameron said in a speech to Parliament.

The decision to hold parts of the inquiry in secret angered human rights groups. "This inquiry is welcome but the devil is in the detail: The idea that this inquiry must be heard in private is misguided and wrong," said Clive Stafford Smith, human rights lawyer for the group Reprieve.

But Cameron said that full transparency was not possible and that intelligence officers would be allowed to testify behind closed doors.

The British prime minister was careful not to say anything Tuesday that might damage his government's relations with Washington.

He said the report would address the "concerns of our allies," and he stressed the importance of "the vast two-way benefit this U.S.-U.K. relationship has brought in disrupting terrorist plots and saving lives."