The UK tolerated "inexcusable" treatment of detainees by the US during the war on terror following the 9/11 attacks in 2001, a parliamentary committee has found.
The Intelligence and Security Committee found no "smoking gun" indicating that security and intelligence agencies had a policy of deliberately overlooking reports of mistreatment and no evidence that UK officers directly carried out physical mistreatment of detainees.
But it said it was "beyond doubt" that British intelligence agencies knew at an early stage that the US was mistreating detainees.
And "more could have been done" by both security agencies and Government ministers in the UK to try to influence US behaviour, the report found.
In 232 cases, UK personnel continued to supply questions or intelligence to allies after they knew or suspected mistreatment, said the ISC.
And in 198 cases, they received intelligence obtained from detainees who they knew or should have suspected had been mistreated.
There were at least 38 cases in 2002 alone of British officers witnessing or hearing about mistreatment.
The committee rejected agencies' claims that these amounts to no more than "isolated incidents", stating: "They may have been isolated incidents to the individual officer witnessing them, but they cannot be considered 'isolated' to those in Head Office.
"It is difficult to comprehend how those at the top of the office did not recognise the pattern of mistreatment by the US.
"That the US, and others, were mistreating detainees is beyond doubt, as is the fact that the Agencies and Defence Intelligence were aware of this at an early point."
The report found three cases when the Secret Intelligence Service (MI6) or MI5 made or offered payment towards the "extraordinary rendition" of detainees to states where they were under real risk of torture or cruel, inhuman or degrading treatment.
In 28 cases, UK agencies suggested, planned or agreed to rendition operations, in a further 22 SIS or MI5 provided intelligence to enable a rendition operation and in 23 more they failed to take action to prevent a rendition.
The report also found evidence of UK officers making verbal threats in nine cases, as well as two cases in which UK personnel were party to mistreatment administered by others.
One of these has yet to be fully investigated and the committee raised the question of whether it should now be reopened.
There was no evidence that any US rendition flight transited the UK with a detainee on board, but two detainees are known to have transited through the Indian Ocean UK territory of Diego Garcia, where records of the conditions under which they were held are "woefully inadequate".
"In our view the UK tolerated actions, and took others, that we regard as inexcusable," said committee chairman and former attorney general Dominic Grieve.
"That being said, we have found no 'smoking gun' to indicate that the agencies deliberately overlooked reports of mistreatment and rendition by the US as a matter of institutional policy.
"The evidence instead suggests a difficult balancing act: the agencies were the junior partner with limited influence, and concerned not to upset their US counterparts in case they lost access to intelligence from detainees that might be vital in preventing an attack on the UK."
Mr Grieve acknowledged the pressure the agencies were operating under at a time when they feared an attack on the scale of 9/11 could be imminent in the UK.
"We wish to be absolutely clear that we do not seek to blame individual officers acting under immense pressure," he said.
But he added: "More could have been done at an agency and ministerial level to seek to influence US behaviour.
"More could also have been done to distance themselves from mistreatment of detainees."