LONDON Libyan Foreign Minister Moussa Koussa, one of Muammar Gaddafi's closest advisers and a former spy chief, defected and flew to Britain on Wednesday in protest at attacks by Gaddafi forces on civilians, a friend said.
A British government source described his resignation as "a significant blow" to Gaddafi and Koussa's predecessor at the ministry said he was "part of the regime's spinal cord ... Koussa is a pillar of the temple."
Koussa is the latest minister to defect after the revolt against Gaddafi's 41-year rule erupted last month and Western-led air strikes began to pound Libyan tanks and artillery being used against rebels fighting to hold territory.
"Koussa 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," a British Foreign Office spokesman said in a statement.
He crossed the border into Tunisia on Monday and flew from there into Farnborough, a business airport in southeast England.
"He travelled here under his own free will. He has told us he is resigning his post," the spokesman said.
Noman Benotman, a friend and analyst at Britain's Quilliam think tank, said Koussa was "seeking refuge" in Britain. "He has defected from the regime," he said. "He wasn't happy at all. He doesn't support the government attacks on civilians."
A Libyan spokesman said Koussa -- who was involved in talks that led to the Libyan convicted of the bombing of an airliner over Lockerbie, Scotland, being released from a British jail -- had not defected and was traveling on a diplomatic mission.
Koussa, Western-educated and English speaking, was the architect of a dramatic shift in Libya's foreign policy that brought the oil-producing desert state back to the international community after years of sanctions.
A potential benefit to the West from his defection is that if he decides to share his knowledge he could reveal valuable information about how the Gaddafi administration functions and the weak points that could be exploited to bring Gaddafi down.
British officials were also keen to establish what role, if any, Koussa wished to play in the anti-Gaddafi coalition. Some of other ministers and ambassadors who have resigned since the revolt started in February have joined the opposition.
A Western diplomat said Koussa's defection was significant because it sent a message to other people at Gaddafi's side that they could still defect, even if they were associated with the bloody crackdown on Gaddafi's opponents.
"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," Britain's Foreign Office said.
However, despite Koussa's influential position, Gaddafi's innermost circle is made up principally of his sons and people with family ties, and their loyalty is likely to be more robust.
Geoff Porter, an independent analyst on North Africa who has testified on Libya in the U.S. Congress, said Koussa's defection was one of the first signs the Gaddafi elite was fracturing.
"So while (Koussa's) ... departure is a sign that things are bad in the Gaddafi camp, it is also a sign that the Gaddafi camp will drift toward extremism, nihilism and acute violence."
Koussa's predecessor Abdurrahman Mohamed Shalgham, a former U.N. envoy who renounced the Libyan leadership, told Al Jazeera news channel that Koussa was the Gaddafi's government's "black box" and with his defection, it had reached its end.
Earlier this month, U.S. Secretary of State Hillary Clinton told ABC television network she was aware that people close to Gaddafi had been trying to make contact.
British Foreign Secretary William Hague has kept in contact with Koussa during the mounting crisis in Libya.
Hague told the BBC last month he had called the minister the previous day "because you still have to communicate to them directly, personally: this situation is unacceptable."
Earlier on Wednesday, Britain said it was expelling five Libyan diplomats to protest at Libya's actions and because they could pose a threat to national security.
(Additional reporting by Christian Lowe, Souhail Karam, Adrian Croft, Maria Golovnina, Aly Eldaly and Marie-Louise Gumuchian; Editing by Alison Williams)