The move was "a significant blow" to Gaddafi, a British government source told Reuters.
Koussa is one of the most senior members of Gaddafi's inner circle to defect -- a major setback for the Libyan leader who faces a revolt against his 41-year rule in the North African oil producing desert state as well as Western air strikes.
Koussa, who was involved in talks that led to the freeing by the British government of the man convicted over the 1988 Lockerbie bombing, is resigning his post and the British government said it hoped more senior figures would join him.
"He travelled here under his own free will. He has told us he is resigning his post," a Foreign Office spokesman said in a statement. "We are discussing this with him and we will release further detail in due course."
He was reported to be being debriefed by British intelligence and foreign ministry officials.
"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," the spokesman 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," the Foreign Office said.
Koussa arrived at Farnborough airport in southern England on a flight from Tunisia, Britain's Foreign Office said.
Koussa is the highest profile of a number of Libyan ministers and ambassadors who have resigned in recent weeks, some of them joining the opposition to Gaddafi.
The British government source described the decision by Koussa, as "clearly a significant blow to the Gaddafi regime."
Noman Benotman, a friend and senior analyst at Britain's Quilliam think tank, said Koussa had defected.
"He wasn't happy at all. He doesn't support the government attacks on civilians," he said. "He's seeking refuge in Britain and hopes he will be treated well."
The Libyan government had said Koussa was traveling on a diplomatic mission and denied he had defected after Tunisia's news agency reported he had flown to London from Tunisia.
Tunisia's TAP news agency had reported on Monday that Koussa had crossed into Tunisia from Libya. TAP said on Wednesday he took off from the Tunisian airport of Djerba, bound for Britain.
Koussa was the architect of a dramatic shift in Libya's foreign policy that brought the country back to the international community after years of sanctions.
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 Libyan foreign 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 the Libyan government's actions and because they could pose a threat to national security.
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