Libya's foreign minister has not been offered immunity after his unexpected arrival in Britain, London said Thursday, while urging other members of Muammer Gaddafi "crumbling" regime to quit.

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, British Foreign Secretary William Hague said.

He was now in talks with British officials but Hague stressed that Kussa, who has been accused of masterminding the 1988 Lockerbie bombing, had not been offered immunity from prosecution.

"Mussa Kussa is not being offered any immunity from British or international justice," Hague told reporters, saying he was being interviewed "voluntarily".

Britain has repeatedly said that Gaddafi and his closest associates should face the International Criminal Court, a call echoed by the rebels fighting the Libyan leader's 42-year-old regime.

A government official said there were no charges pending against Kussa.

Hague confirmed the Libyan minister had told British officials on his arrival that he was resigning his job, adding that he was now being held in a "secure place" while they discussed "his options and our options".

A senior administration official in the United States, which with Britain and France has led air strikes to protect civilians from Kadhafi's forces, told AFP that Kussa's resignation showed "the writing's on the wall" for the regime.

Hague echoed this, saying: "His resignation shows that Kadhafi's regime, which has already seen significant defections to the opposition, is fragmented, under pressure and crumbling from within.

"Gaddafi must be asking himself who will be the next to abandon him."

He urged others to follow Kussa's lead. "The Gaddafi regime has lost all legitimacy and today I renew our calls for those around him to abandon and to unite in support of a better future for their country."

One defector, former immigration minister Ali Errishi, told France 24 television that Kussa's move showed "the regime's days are numbered".

Kussa, 59, flew to Britain from Tunisia, where he had spent two days on what Tripoli had officially described as a private visit.

In its first admission of the defection, Libya's government on Thursday shrugged off his departure, saying Kadhafi's regime "does not depend on individuals".

Hague said Kussa had been his main contact in Tripoli but admitted he only knew a "very short time in advance" that he would be coming to Britain.

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," former British foreign secretary Jack Straw told BBC radio.

"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 Pan Am 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 also expelled from Britain in 1980, just months after he was appointed ambassador to London, after telling a journalist he approved of killing "enemies" of the Libyan regime.

On Wednesday, Hague announced the expulsion of five diplomats at the Libyan embassy in London because of the threat they posed to opposition dissidents.