The UK says it has not offered Libyan Foreign Minister Moussa Koussa immunity from prosecution following his unexpected arrival in the country.
Foreign Secretary William Hague said Mr Koussa had quit, and Muammar Gaddafi's regime was "crumbling from within".
UK officials are questioning Mr Koussa amid opposition claims that he helped plan the 1988 Lockerbie plane bombing.
In Libya, a rebel commander says pro-Gaddafi forces have intensified attacks on the besieged city of Misrata.
Saadoun al-Misrati told the BBC the city had been hit by shells and missiles almost without a break for the past 24 hours.
Further east, the fighting continues, with rebels pushing pro-Gaddafi forces back to the outskirts of Brega, where they have once again reached deadlock.
The BBC's Christian Fraser in eastern Libya says each time the rebels attempt to advance past a fork in the main road, they come under a barrage of rocket attacks.
The insurgents' light weaponry is no match for the longer range weapons of Col Gaddafi's forces, says our correspondent.
Mr Hague said Mr Koussa had flown to the UK of his own free will late on Wednesday.
"His resignation shows that Gaddafi's regime, which has already seen significant defections to the opposition, is fragmented, under pressure and crumbling from within," he told reporters.
"Gaddafi must be asking himself who will be the next to abandon him."
Since the uprising began in February, the Gaddafi regime has been hit by a number of resignations, including the interior minister, justice minister and several ambassadors.
And rumours have started swirling in Tripoli of a string of other defections.
Libya's government initially denied that Mr Koussa had defected, and later sought to play down the incident by saying the regime was bigger than any individual.
"He said he needed some medical treatment for a few days in Tunisia, but we expected him to come back," said spokesman Moussa Ibrahim.
Scottish prosecutors say they have made a formal request to interview Mr Koussa over the bombing of a jumbo jet above the town of Lockerbie in 1988.
He was head of Libya's foreign intelligence at the time of the attack, which left 270 people dead.
Meanwhile, the debate rages over what help western governments can give to the rebels.
Maj Gen Suleiman Mahmoud, the second-in-command for the rebels, told the BBC that rebel forces needed time, patience and help to organise themselves.
"Our problem [is] we need help - communication, radios, we need weapons," he said, adding that the rebels had a strategy but fighters did not always obey orders.
American Defence Secretary Robert Gates reiterated that there would be "no US boots" on the ground, but declined to comment on reports that the government was backing covert CIA operations on the ground.
The US and the UK have suggested the UN resolution authorising international action in Libya could also permit the supply of weapons.
However France - which helped push through the UN resolution authorising "all necessary measures" to protect civilians from Col Gaddafi's forces - says it is not planning to arm the rebels.
On Thursday, French Defence Minister Gerard Longuet said such assistance was "not compatible" with the resolution.
Nato, which took sole command of international air operations over Libya on Thursday, said it was strongly opposed to arming the rebels.
The alliance also said it was investigating reports of civilian casualties in Western air strikes on Tripoli.
Earlier, the top Vatican official in the Libyan capital, citing witnesses, said 40 civilians had been killed in strikes by Western forces on the city.
The uprising against Col Gaddafi's regime began on 16 February, with protesters spurred on by the ousting of neighbouring long-time rulers in Tunisia and Egypt.