British intelligence officers were aware of inappropriate interrogation techniques and mistreatment of prisoners by foreign partners, The Detainee Inquiry found after a three-year probe.
The agents were reluctant to do anything which would jeopardise relations with allies such as the United States, it said.
Former appeal court judge Peter Gibson, who led the inquiry, told reporters: "It does appear from the documents that the United Kingdom may have been inappropriately involved in some renditions.
"That is a very serious matter. And no doubt any future inquiry would want to look at that."
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Those questions will now be addressed by parliament's Intelligence and Security Committee, with a report due by the end of 2014.
Government minister Kenneth Clarke told the House of Commons that it was a matter of "sincere regret" if "mistakes and failures were made".
"It is now clear that our agencies and their staff were in some respects not prepared for the extreme demands suddenly placed upon them," he said.
"There is some damage to our reputation, which prides itself as a beacon of justice, human rights and the rule of law."
It found evidence that the British government or its agencies may have been involved in some cases where suspects were illegally transferred from one jurisdiction to another.
Gibson and his team reviewed 20,000 top-secret documents looking at interrogation and treatment issues; rendition; training and guidance; and policy and communications.
"Documents indicate that in some instances UK intelligence officers were aware of inappropriate interrogation techniques and mistreatment or allegations of mistreatment of some detainees by liaison partners from other countries," it concluded.