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 ministers in Tony Blair's Government 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 (CIDT).
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.
In a written statement responding to the report, Prime Minister Theresa May acknowledged that the alleged threats to detainees by intelligence officers and troops was "clearly unacceptable", adding that improvements have since been made to provide enhanced oversight of operations where this is a risk.
"With the benefit of hindsight, it is clear that UK personnel were working within a new and challenging operating environment for which, in some cases, they were not prepared," said Mrs May.
"It took too long to recognise that guidance and training for staff was inadequate and too long to understand fully and take appropriate action on the risks arising from our engagement with international partners on detainee issues.
"The agencies responded to what they thought were isolated allegations and incidents of mistreatment, but the ISC concludes that they should have realised the extent to which others were using unacceptable practices as part of a systematic programme. The agencies acknowledge that they did not fully understand this quickly enough and they regret not doing so."
She added: "Working closely with international partners is an essential part of keeping this country and its people safe. In doing so UK personnel seek assurances from those countries on their treatment of individuals and make clear the UK's position on torture and CIDT.
"Detainee-related work remains important and at times difficult, but intelligence and Armed Forces personnel are now much better placed to meet that challenge."
The report found 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."
In a three-year inquiry, the ISC reviewed 40,000 documents and interviewed former detainees and three ex-officials.
But it was denied access to intelligence officers who were involved in the events.
Mr Grieve said the Government's refusal to allow agents to give oral evidence forced the committee "reluctantly" to cut its investigation short.