Nicknamed the "envoy of death" by his enemies, Musa Kusa was once among the most reviled men in Britain, accused of sending hitmen around the world to kill opponents of the Gaddafi regime.
But in a demonstration of his nation's return from the cold, he led the team that agreed, in the refined surroundings of a Pall Mall club, that Libya would stop developing weapons of mass destruction.
In return for their co-operation, Libya will be looking for the lifting of trade sanctions imposed after Lockerbie.
Mr Kusa was expelled as head of the Libyan diplomatic mission in 1980 after backing the murder of Libyan dissidents. In an interview at the time, he said: "The revolutionary committees have decided last night to kill two more people in the United Kingdom. I approve of this."
He went on to warn that Libya would give active backing to the IRA if Britain refused to hand over opponents of Colonel Muammar Gaddafi sheltering in Britain.
The threats led the Tory government to announce, to cheers in the Commons, that Mr Kusa was being ordered out of the country immediately.
Eight years later, Mr Kusa was a suspect in the worst terrorist attack on British soil, the Lockerbie bombing. The French also accused him of being involved in the blowing up of a French airliner over the Sahara in 1989 with the loss of 170 lives.
In 1995, MI5 warned that Mr Kusa, as head of Libya's intelligence service, was in charge of pro-Gaddafi agents operating in Britain and responsible for state-sponsored terrorism around the globe. The former CIA head of counter-terrorism Vince Cannistraro has said that he has "blood on his hands all round the world".
The diplomatic atmosphere changed dramatically after the al-Qa'ida attacks on New York and Washington, with Libya emerging as a useful ally for the West.
The first sign of old enmities being buried came early last year when it emerged that Mr Kusa was acting as Colonel Gaddafi's special envoy in talks on compensation for the relatives of the Lockerbie victims.
His involvement upset their families and raised hackles within the intelligence services, but the Foreign Office appeared to believe it was more important to focus on the bigger diplomatic picture.
After a deal on Lockerbie was reached last year, Mr Kusa, now 56, moved on to wider issues, contacting London nine months ago. As part of the drive to bring Libya back into the world community and to get trade sanctions lifted, he is understood to have produced intelligence information about terrorists operating in Britain, continental Europe and North Africa in meetings with British and American agents.
Against that background he sat down on Thursday in the Travellers' Club - founded 184 years ago for gentlemen to swap tales of foreign adventures - with senior figures from the Foreign Office and MI6. Joining him was Mohamed Abdul Quasim al-Zwai, the Libyan ambassador to London, and Ali Abdalate, the Libyan ambassador to Rome.
During the six-hour session, which stretched late into the afternoon, the Libyan and British teams drank only mineral water. After shaking hands on the outline of a deal, Tony Blair spoke by telephone to Colonel Gaddafi- the first time the men have spoken.
A day later, the historic agreement brokered by "the envoy of death" was revealed to a stunned world.Reuse content