Moussa Koussa, the Libyan foreign minister, has defected to the United Kingdom, the British foreign ministry has said.
The ministry said in a statement that Koussa had arrived at Farnborough Airport, in the south of England, on a flight from Tunisia on Wednesday.
"He travelled here under his own free will. He has told us that he is resigning his post. We are discussing this with him and we will release further details in due course," the statement said.
"We encourage those around Gaddafi to abandon him and embrace a better future for Libya that allows political transition and real reform that meets the aspirations of the Libyan people."
It added that Koussa was one of the most senior officials in Gaddafi's government with a role to represent it internationally, which is "something that he is no longer willing to do".
Tunisia's TAP news agency said on Monday that Koussa had crossed over into Tunisia from Libya.
A government spokesman in the Libyan capital Tripoli had earlier denied speculation that he had defected.
"He is on a diplomatic mission," Mussa Ibrahim, the spokesman, said. He gave no further details.
Earlier on Wednesday, the British government announced the expulsion of Libya's military attache and four other diplomats in protest and for intimidating opposition groups in London.
A government source quoted by Reuters said the diplomats, believed to be supporters of Libyan leader Muammar Gaddafi, have been given seven days to leave.
William Hague, the British foreign minister, told legislators the move was to "underline our grave concern at the regime's behaviour".
"... we have today taken steps to expel five diplomats at the Libyan embassy in London, including the military attache," he said in parliament on Wednesday.
"The government also judged that, were those individuals to remain in Britain, they could pose a threat to our security."
"We believe they are among the strongest Gaddafi supporters in the embassy, that they have put pressure on Libyan opposition and student groups in the UK and that there is a risk of damage to UK national security if they remain"
William Hague, British foreign minister
Hague also announced that a British diplomatic mission led by senior diplomat Christopher Prentice had visited the rebel-held city of Benghazi earlier this week, and met key opposition groups including Mustafa Abdel Jalil, the head of the rebel Libyan National Council.
Britain has long treated Libya as a rogue state. The 1984 shooting of a London policewoman from inside the Libyan embassy, the Libyan arming of IRA guerrillas in Northern Ireland and the 1988 Lockerbie airline bombing over Scotland, for which a Libyan was convicted, contributed to Gaddafi being branded a pariah.
A foreign office spokesman, the expelled diplomats were thought to be strong Gaddafi supporters.
"We won't go into details on their activities," the spokesman said.
"But we believe they are among the strongest Gaddafi supporters in the embassy, that they have put pressure on Libyan opposition and student groups in the UK and that there is a risk of damage to UK national security if they remain."
Britain hosted an international conference on Tuesday that piled pressure on Gaddafi to quit and pledged to continue military action against his forces until he complies with a UN resolution to protect civilians.
At the London meeting, the question of arming Libyan rebels moved up the international agenda, although both Britain and the United States said they had taken no decision to supply arms.
On Wednesday, David Cameron, the British prime minister, repeated that line, adding that UN resolution 1973 allowed all necessary measures to protect civilians.
"Our view is that this would not necessarily rule out the provision of assistance to those protecting civilians in certain circumstances," Cameron told parliament.
"So ... we do not rule it out but we have not taken the decision to do so."
Expressing his reservations, British foreign minister Hague said introducing new weapons into a conflict could have "unforeseeable and unknown consequences".
"Such considerations would have to be very carefully weighed before the government changed its policy on this matter," he added.