Diplomatic correspondent, BBC News
The British government hates having to apologise to Members of Parliament for telling them something which is not true.
David Miliband apologised for two "extraordinary rendition" flights
When they have been misled on different occasions, over several years, and it turns out Britain's number one ally is to blame, it makes it even more uncomfortable.
To make matters worse, doubters never had accepted American denials that any of the CIA's "extraordinary rendition" flights had touched down on British territory.
Human rights charities and campaigners insisted they had credible evidence that the British overseas territory of Diego Garcia had been used as part of the US's secret movements of suspects abducted abroad.
Now the Foreign Secretary, David Miliband, has had to tell those critics and sceptics their disbelief was well-founded.
This is, in his words, "a most serious matter".
Trust is at the very heart of the relationship between the governments in London and Washington.
Mr Miliband insisted on that throughout his statement to the House of Commons, throughout his answers to MPs' questions, and in a series of interviews designed to show a repentant government in full "apology mode".
He made little attempt to conceal the political damage this will do in Britain.
The foreign secretary told Parliament that British officials, including government lawyers, will now go to the US.
He suggested that the trawl of CIA flights - to check for evidence they may have used UK airspace or territory - needs to continue.
When the foreign secretary was asked by one MP: "What's so special about a relationship in which one of the partners abuses trust and respect?", he found himself on the defensive again.
"The breach is not the defining element of our relationship with the United States," he said.
"I don't believe the relationship is based on deceit, but on shared values and commitments."
The problem with that, as other MPs pointed out, is that Britain says it does not share some crucial American values.
Two "rendition" flights refuelled on territory of Diego Garcia
President Bush defends both the practice of "extraordinary rendition" and of "water-boarding", both of which Britain rejects.
More than that, Britain may have been put in the position of breaking its own international commitments to reject all forms of torture, even if it played an unwitting role in the flights through Diego Garcia.
Mr Miliband was quick to underline that he did not believe, in these cases, either detainee was en route to destinations where they were subject to torture.
Mr Miliband also stressed the importance of the legal agreements between Britain and the US over Diego Garcia.
They are contained in letters of 1966 and 1976 which, Mr Miliband insisted, have the force of a treaty, and are legally binding.
He implied, without saying it, that the treaty may have been broken by the Americans.
The result of all this? Those in Britain most hostile to the closeness between President George Bush and the government will feel strengthened in their argument that it is Washington which calls the shots.
No wonder the foreign secretary wanted to emphasise the positive in that relationship: "One of enduring value for the life of the people of this country."