Libya's foreign minister was being questioned by British officials Thursday after his surprise defection, viewed by opponents of Libyan leader Muammar Gaddafi as a sign that his regime was crumbling.
Mussa Kussa, a former head of Libyan intelligence and one-time ambassador to Britain, arrived "under his own free will" at Farnborough airport southwest of London on Wednesday, the Foreign Office said.
"He has told us that he is resigning his post. We are discussing this with him and we will release further detail in due course," it said in a statement.
It added: "Mussa Kussa is one of the most senior figures in Gaddafi's government and his role was to represent the regime internationally, something that he is no longer willing to do."
Britain urged other Libyans to abandon Gaddafi, whose forces are battling rebels who want an end to his four-decade rule.
"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," the Foreign Office said.
Kussa, 59, flew to Britain from Tunisia, where he had spent two days on what Tripoli had officially described as a private visit.
When he was reported to be on his way to London, the Libyan authorities said he was on a "diplomatic mission".
The United States, which with Britain and France has led military action to enforce a no-fly zone over Libya to protect civilians from the fighting, hailed Kussa's departure as a major blow to Gaddafi's regime.
"This is a very significant defection and an indication that people around Gaddafi think the writing's on the wall," a senior US administration official said.
Former British foreign secretary Jack Straw suggested the defection could be a "tipping point".
"Mussa Kussa's apparent defection -- certainly his unscheduled visit here -- will be a very important factor in just adding to the weight against the Gaddafi regime and tipping the balance against him," he told BBC radio.
Oliver Miles, a former British ambassador to Libya, said it would make it easier for others to defect, adding: "That's what we should be trying to encourage."
Several ministers and senior military officers have already abandoned Gaddafi and one of them, former immigration minister Ali Errishi, told France 24 television that Kussa's move showed "the regime's days are numbered".
As head of Libyan intelligence for 15 years before his appointment as foreign minister in March 2009, Kussa is credited with convincing Gaddafi to dismantle his nuclear weapons programme and renew ties with the West.
"He has occupied a key position at the heart of the intelligence and security apparatus of the Gaddafi regime," Straw remarked.
"And I am in absolutely no doubt that he played a fundamentally important role in getting Gaddafi to agree to give up his nuclear weapons programme and his chemical weapons programme."
However, Kussa has also been named as the architect of the 1988 airline bombing over the Scottish town of Lockerbie, which killed 270 people.
Libyan agent Abdelbaset Ali Mohmet al-Megrahi was jailed for the attack but was granted compassionate release from a Scottish jail in August 2009.
Kussa was expelled from Britain in 1980, just months after being appointed ambassador to London, after telling a journalist he approved of killing "enemies" of the Libyan regime.
British Foreign Secretary William Hague announced Wednesday that five diplomats at the Libyan embassy in London, including the military attache, were being expelled because of the threat they pose to opposition dissidents.
A Foreign Office spokesman said: "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."