Libyan Foreign Minister Moussa Koussa is in Britain and "no longer willing" to work for Colonel Muammar Gaddafi's regime, the UK Foreign Office says.
He flew in from Tunisia on a non-commercial flight and was questioned for several hours by British officials.
"He travelled here under his own free will. He has told us he is resigning," said a Foreign Office spokesman.
His apparent defection comes as rebels in Libya are retreating from former strongholds along the eastern coast.
The rebels have now lost the key oil port of Ras Lanuf and the nearby town of Bin Jawad, and are also in full retreat from Brega.
In the west, the rebel-held town of Misrata is still reportedly coming under attack from pro-Gaddafi troops, reports say.
'Own free will'
A British Foreign Office spokesperson said: "We can confirm that Moussa Koussa arrived at Farnborough Airport on 30 March from Tunisia.
"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.
"Moussa 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.
"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 in London called on other members of the Libyan government to abandon Colonel Gaddafi.
UK intelligence officials hope that his deep knowledge of the Libyan regime will help bring about its early end, says the BBC's diplomatic correspondent Humphrey Hawksley.
Mr Koussa arrived in London on what is believed to have been a British military plane, our correspondent adds.
Libya foreign minister quits