Mr. Koussa’s flight to Britain came three decades after he was expelled from that country when he told a reporter for The Times of London that he supported Libya’s practice of hunting down and killing Libyan opponents of Colonel Qaddafi around the world.

“I approve of this,” the young diplomat declared in that 1980 interview.

On the face of it, Britain would appear to be the last place Mr. Koussa might be expected to seek refuge. In addition to the Pan Am 103 bombing, the Libyan government supplied arms and explosives to the Irish Republican Army and other terrorist groups, and was responsible for the killing of a British policewoman shot from inside the Libyan Embassy in London in 1984. On Friday, a headline in The Daily Mail called him Colonel Qaddafi’s “Fingernail-Puller-in-Chief.”

But Mr. Koussa, by all accounts a canny operator who does not act rashly, clearly calculated that he was better off taking his chances in Britain than sticking with Colonel Qaddafi.

“Any defection is a bet on which side is going to win,” said Paul R. Pillar, a former official of the Central Intelligence Agency who has met Mr. Koussa. “I assume he’s banking on the leverage he has with his inside information, even without a formal grant of immunity. It’s a plea-bargaining situation.”

Mr. Koussa’s calculation is undoubtedly based, too, on the sterling connections to British and American intelligence officials that he forged as Libya’s central negotiator when Colonel Qaddafi decided in 2004 to give up efforts to develop nuclear, chemical and biological weapons. He came across not as a thug but as an urbane and worldly figure “who would not have looked out of place as a Western ambassador,” Mr. Pillar said.

Diplomatic cables obtained by WikiLeaks show that in recent years, Mr. Koussa remade himself as the reasonable face of a ruthless and erratic government, meeting with American diplomats to discuss terrorist threats in North Africa, the plight of Sudanese refugees in Darfur, the streamlining of visas for American tourists and even human rights inside Libya.

A graduate of Michigan State University now in his early 60s, Mr. Koussa “is the rare Libyan official who embodies a combination of intellectual acumen, operational ability and political weight,” said a May 2009 cable from the American Embassy in Tripoli. The cables also portray him as a mentor to two of Colonel Qaddafi’s sons: Muatassim, who serves as national security adviser, and Seif al-Islam, who was once seen as a proponent of reform. (Seif al-Islam has now proposed taking power from his father, a plan Muatassim is apparently resisting.)