His insistence came as Downing Street made clear that Gordon Brown had not endorsed the decision by Justice Secretary Kenny MacAskill to release the Libyan on compassionate grounds.

A source stressed that in “respecting” his decision the Prime Minister was not agreeing with it. He told The Herald Mr Brown would have respected the Scottish Government’s judgment “whatever the decision”.

The source also made clear that despite the anger expressed in some quarters in America, the Prime Minister was still of the view the decision to release Megrahi had not damaged UK-US relations. “We have a very strong and deep special relationship that will survive ups and downs,” he added.

Nonetheless, the anger on the other side of the Atlantic does not appear to be abating.

Frank Lautenberg, a Democratic senator, is demanding a Senate investigation into allegations an oil deal was brokered. He has written to Senator John Kerry, Chairman of the Senate Foreign Relations Committee, asking for a congressional panel to be established to “expose the truth” and “uncover whether justice took a back seat to commercial interests”.

He wants to know: whether oil and commercial interests led the UK Government to include Megrahi in the prisoner transfer agreement when it had previously sought to exclude him; if Megrahi’s release violated an agreement between Britain and America; whether oil contracts coloured the UK Government’s actions, including its failure to object to the Libyan’s release; and whether commercial interests played a role in the decision to set Megrahi free.

After initially decrying Mr MacAskill’s decision, the US administration has maintained a diplomatic silence in spite of media and political anger, particularly from New York and New Jersey, where many of the Lockerbie victims came from.

Sir Christopher, speaking about his time as ambassador in Washington during the 1998 negotiations over Megrahi’s trial, said: “One thing I do remember very, very clearly was that it was very important to them [the US] to get a commitment out of us that if Megrahi and the other guy were found guilty, they would serve the full term of their sentence in a UK jail.

“That was a vital selling point for the relatives and friends of the Americans who died in the blowing up of the Pan Am flight.” He added that this was the “clear political and diplomatic understanding” the US had.

However, the Foreign Office responded: “In our discussions with the US Government, we made it clear that the imprisonment of the accused in Scotland was always without prejudice to Scottish ministers taking decisions under Scottish law.”

Elsewhere, Mark Zaid, an American lawyer representing about 50 families of Lockerbie victims, is preparing a lawsuit to force the US Government to release documents relating to the case.

He said he hoped to obtain papers that would “reveal any backdoor arrangements or agreements or discussions between our two governments”.

At Westminster, the Conservatives have tabled a raft of questions to ministers over the Megrahi case including, among other things, what ministerial discussions there were with Colonel Gaddafi and his son Saif al Islam over oil and gas contracts.