British concerns did not appear to "materially" affect US actions in its "war on terror", the UK's intelligence and security committee has said.
The committee, which reports to the prime minister, was probing possible UK involvement in rendition flights.
It said America's "lack of regard" for UK concerns had "serious implications" for future intelligence relations.In response, the UK government said the countries' intelligence relationship was "close" and "must continue".
The committee said it had found no evidence that the UK was directly involved in rendition flights - the transportation of terror suspects to foreign prisons where they could face torture.
But Britain's security services had "inadvertently" helped in one case after the US ignored caveats placed on supplied information.
It said ministerial approval should be required in future in such cases and a complete ban placed on approvals for renditions which could lead to suspects being held in secret prisons.
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The committee also levelled criticism at the government over inadequate record-keeping.
Committee chairman, former Northern Ireland Secretary Paul Murphy, said: "Our inquiry has not been helped by the fact that government departments have had such difficulty in establishing the facts from their own records in relation to requests to conduct renditions through UK airspace.
"This is a matter of fundamental liberty, and we recommend that the government ensure that proper searchable records are kept in the future."
The cross-party committee said "routine" evidence sharing in the case of two British residents in Ghana in 2002 "indirectly and inadvertently" led to their rendition.
The rendition program has revealed aspects of this usually close relationship that are surprising and concerning
Intelligence and security committee report
Bisher al-Rawi and Jamil el-Banna were flown by the CIA first to Afghanistan and then Guantanamo Bay, where el-Banna is still being held.
The committee said the UK services "used caveats specifically prohibiting any action being taken" when they handed over the intelligence on the men.
It says the UK security services did not foresee that the US authorities would disregard the caveats, given that they had honoured the caveat system for the past 20 years.
"This case shows a lack of regard on the part of the US for UK concerns - despite strong protests - and that has serious implications for the intelligence relationship," the report concluded.
"In fighting international terrorism it is clear that the US will take whatever action it deems is necessary, within US law, to protect its national security," it said.
"Although the US may take note of UK protests and concerns, it does not appear materially to affect their strategy; the rendition program has revealed aspects of this usually close relationship that are surprising and concerning," the report warned.
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The committee also criticized the UK security services for failing to react quickly enough to the change in American approach and start to use "greater caution".
It said British intelligence had been briefed in November 2001 about new powers enabling the US authorities to arrest and detain suspected terrorists worldwide.
But officers were "skeptical about the supposed new powers because at the time there was a great deal of 'tough talk' being used at many levels of the US administration," the report says.
The report says British intelligence officers "should have noted the significance of these events and reported them to ministers".
Procedures had now been tightened and provided a "reasonable level of confidence" in most cases, the committee concluded.
But it recommended two further changes to strengthen safeguards.
"First, where despite the use of caveats and assurances there remains a real possibility that sharing intelligence with foreign liaison services might result in torture or mistreatment...we recommend that ministerial approval should be sought in all such cases.
"Secondly, the Committee considers that 'secret detention', without legal or other representation, is of itself mistreatment.
"Therefore, where there is a real possibility of 'Rendition to Detention' to a secret facility, even if it would be for a limited time, we consider that approval must never be given."
The British government said the report supported its own "repeated assurance that there is no evidence to suggest that renditions have been conducted through the UK without our permission, or in contravention of our obligations under domestic and international law".
The government also stressed the "importance of the UK's international intelligence relationships, particularly with the United States, in countering the threat from international terrorism", a reality that the Committee itself acknowledges.
The government also said that British intelligence staff did inform ministers of exchanges with US counterparts in November 2001 about the US plans to arrest and detain suspected terrorist suspects worldwide.
Conservative MP Andrew Tyrie attacked Gordon Brown for failing to condemn "extraordinary rendition" - a policy he said was now "being vigorously condemned by many in Washington, both Republican and Democrat".
"This is the price the prime minister of Britain is paying for remaining so wedded to the policy he has inherited of 'hugging the Americans close'," added Mr Tyrie, who co-founded an all-party group on rendition and gave evidence to the committee.
Amnesty International said it welcomed the report's acknowledgment that secret detention amounts to 'mistreatment' and that the UK "must never give approval to measures leading to it".
UK campaigns director Tim Hancock said: "Rendition is the fast track to Guantanamo and we need to see the government unequivocally condemning all renditions and secret detentions.
"Gordon Brown's government should now allow a fully independent inquiry into rendition and also immediately call for all UK residents at Guantanamo to be properly tried or released and returned to the UK."
© BBC MMVII