The Libyan government has confirmed that foreign minister Moussa Koussa has resigned, but is not saying that he has defected.
Mr Koussa flew from Tunisia to Britain on Wednesday, saying he no longer wanted to work for Muammar Gaddafi's government.
British Foreign Secretary William Hague has made it clear Mr Koussa is not being offered immunity from prosecution.
He said Muammar Gaddafi's regime was "crumbling from within".
The BBC reports UK officials are questioning Mr Koussa amid opposition claims that he helped plan the 1988 Lockerbie bombing.
Scottish prosecutors say they have made a formal request to interview Mr Koussa over the bombing of a jumbo jet above the town of Lockerbie in 1988.
He was head of Libya's foreign intelligence at the time of the attack, which left 270 people died.
Mr Hague said Mr Koussa flew to Britain of his own free will late on Wednesday. He arrived at Farnborough Airport in Hampshire on a private charter plane.