JOIN US"His resignation shows that Gaddafi's regime, which has already seen significant defections to the opposition, is fragmented, under pressure and crumbling from within," Hague said as Koussa was being questioned after his arrival.
But in a surprise move, Hague asserted that the Libyan leader was not being offered immunity from British or international justice.
Moussa Koussa is one of the most senior figures in Gaddafi's government and his role was to represent the regime internationally, but during questioning he said that he was no longer willing to represent Gaddafi.
BBC said the Libyan Foreign Minister has arrived in UK on what is believed to have been a British military plane.
But a British foreign office spokesperson said, Koussa who is in early 60s had arrived in Farnborough airport, west of London, last night.
"He told us that he has resigned his post and had come from Tunisia where he had crossed over two days back," the spokesperson said.
Describing the defection as "encouraging signs", Hague said, "We are asking all those around Gaddafi to abandon him and embrace a better future for Libya that allows political transition and real reforms that meet the aspirations of the people.
"Gaddafi must be asking himself who will be the next to abandon him," the British Foreign Secretary said. His arrival in the UK comes as Libyan rebels are reported to be in full retreat from recently captured town along the country's eastern coast.
Libya's Interior and Justice ministers resigned earlier in the conflict and join the rebels fighting in the east.
Libyan exiles in London said that Koussa had been Gaddafi's right hand man for years, running intelligence, and running the Lockerbie bomber negotiations.
Earlier, Tunisian government official said that Koussa was accompanied by Libya's Vice Minister Abdelati Laabidi, who returned to Libya from Tunisia.