By Gordon Corera
BBC News security correspondent
At the heart of the Intelligence and Security Committee's report into rendition is the story of the UK's intense but occasionally difficult relationship with the US when it comes to intelligence sharing and counter-terrorism since 9/11.
MPs found "surprising" aspects of the Anglo-US relationship
The report makes clear just how important the relationship with the US is over counter-terrorism, in the eyes of top British officials.
"It is neither practical, desirable, nor is it in the national interests, for UK agencies to carry out [counter-terrorism] work independently of the US effort," argued the Chief of MI6, Sir John Scarlett, before the committee.
But the ISC report reveals aspects of the usually close Anglo-American intelligence relationship that are "surprising and concerning".
These tensions have centred on the evolving US policy of rendition - the transferring detainees from one country to another, in some cases to stand trial, in other cases to US military detention or even to third countries for interrogation - and its alleged mistreatment.
This policy has meant that for British intelligence "ethical dilemmas are not confined to countries with poor track records on human rights - the UK now has some ethical dilemmas with our closest ally " because of "very different legal guidelines and ethical approaches".
The practice of rendering suspects to trial in the US or elsewhere pre-dated 9/11 and the UK was marginally involved. But after 9/11 it took on a whole new direction and intensity, one which British intelligence only gradually understood.
In the months after 9/11, the atmosphere was, in the words of an MI6 officer "frenetic".
"We were struggling very hard. It felt like trench warfare," explained the former head of MI5 to the committee.
The focus on preventing further attacks meant that significant changes in US policy were not immediately appreciated by British intelligence. When MI6 was given notice of new detention powers by US counterparts, they were sceptical, believing it part of the "tough talk" coming out of Washington post-9/11 rather than marking a major substantive shift.
In fact the shift was real when it came to rendition and detention of terrorist suspects and the committee found that MI6 should have "appreciated the significance of these events and reported them to ministers".
It was only with a number of specific cases in 2002 that it began to dawn on British intelligence just how the US was now operating and that the US was transferring people not just to home countries or the US for trial but to other countries and facilities.
The case of Khalid Sheikh Mohammed, detained in March 2003, seems to have been a watershed for MI5 and MI6's understanding of what the US was up to, raising new questions.
The UK has lost its "inhibitions" about questioning the US
After his arrest, Khalid Sheikh Mohammed had been transferred to one of the so-called CIA "black sites" where he was subjected to practices such as simulated drowning or water-boarding.
From 2004, MI5 and MI6 realised that their existing guidance was not sufficiently detailed to deal with these kind of new questions regarding co-operation and it had to be updated.
In one case in early 2005, MI6 developed an operation that might have produced high-value intelligence on a target but the plan was dropped as it could not be sure of how the target of the operation would be treated and how long they would be detained.
The report also looks at a number of specific cases of alleged British involvement in rendition.
In some cases, it finds nothing to criticise, although in the case of Bisher Al-Rawi and Jamil El-Banna, the detail is particularly illuminating when it comes to US-UK intelligence relations.
Information passed to the US on the men was transmitted with an assurance that no action would be taken on the basis of it. But the men were picked up after travelling from the UK to Africa and eventually transferred to Guantanamo.
The UK expressed its concerns but there was a "lack of regard on the part of the US for UK concerns" as it ignored protests of both MI5 and the government.
US flights passed through UK airspace, the report said
"This has serious implications for the working of the relationship between the US and UK intelligence and security agencies," the committee argues.
Much of the attention has been on the issue of CIA rendition flights transiting through UK airspace and airports.
Here, the committee finds that while there is plenty of evidence of CIA flights coming through - including some which had just carried prisoners - there is no evidence of any flights carrying actual prisoners having passed through the UK, although it notes poor record-keeping in the past.
The report makes clear that British policy when it comes to liaison with foreign services has changed.
"We certainly have inhibitions...greater inhibitions than we once did," testified Eliza Manningham-Buller when she was still head of MI5.
"We would now absolutely say, where is this man? What are you going to do with the information? Where is he being held?"
The report notes that while the US may take note of UK protests over rendition, such concerns do not appear to affect US strategy in any way.
As a result, the UK intelligence community has had to find ways to work round the problems and maintain the exchange of intelligence which everyone agrees is "critical" to UK security.