Scottish prosecutors have asked to interview Muammar Gaddafi's former intelligence chief and foreign minister about the Lockerbie bombing, after his surprise defection to Britain.
Musa Kusa is being debriefed by UK officials and diplomats at an undisclosed "safe location" after arriving on Wednesday night at Farnborough Airport in Hampshire from Tunisia by private plane saying he was no longer willing to work for the dictator.
Prime Minister David Cameron said the decision of one of the most senior members of the Tripoli government to flee Libya told "a compelling story of the desperation and fear right at the top of the crumbling and rotten Gaddafi regime" and said the former foreign minister's defection had dealt a "serious blow" to Gaddafi's authority, urging other members of the dictator's inner circle to abandon him.
Both the PM and Foreign Secretary William Hague stressed that Kusa has not been offered immunity from prosecution in return for information he may be able to provide about Gaddafi's military struggle with rebel forces.
Mr Cameron said: "Let me be clear - Musa Kusa is not being granted immunity. There is no deal of that kind. And the point I would make about the dreadful events over Lockerbie - that investigation is still open and the police and the prosecuting authorities are entirely independent of Government and they should follow their evidence wherever it leads and the Government will assist them in any way possible."
Before becoming foreign minister in 2009, Kusa had been head of Gaddafi's feared intelligence agency since 1994 and was a senior intelligence agent at the time of the 1988 bombing of Pan Am flight 103 over Lockerbie. It was not immediately clear whether prosecutors want to question him as a suspect in planning the atrocity or as someone who is certain to hold vital information because of his position in the Gaddafi regime.
A spokesman for the Scottish Crown Office said: "We have notified the Foreign and Commonwealth Office that the Scottish prosecuting and investigating authorities wish to interview Kusa in connection with the Lockerbie bombing. The investigation into the Lockerbie bombing remains open and we will pursue all relevant lines of inquiry."
Jim Swire, whose daughter Flora was killed in the bombing, said he thought Kusa "knows everything". Dr Swire said: "If Libya was involved in Lockerbie, he can tell us how they carried out the atrocity and why."
Mr Hague said that Kusa is "voluntarily talking" to British officials, but no information was immediately available about whether he had provided useful intelligence about the state of the Gaddafi regime after two weeks of air strikes and the imposition of a no-fly zone by the international coalition. Officials would say only that it is "early days" in his debriefing.
It is thought likely that his questioners - who include diplomats from the UK's Tripoli Embassy who had dealings with him in his former role - will tread carefully at first and take a measured approach to what will be a delicate process of encouraging him to speak openly about what he knows.