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 resigned and the Gaddafi regime was "crumbling from within".British officials are questioning Mr Koussa, a former head of intelligence who was close to Col Gaddafi.
The development comes as Libyan rebels continue to retreat from recently captured towns along the eastern coast.
A column of retreating rebel fighters came under heavy fire between Brega and Ajdabiya on Thursday. The rebels had earlier lost the key oil port of Ras Lanuf and the nearby town of Bin Jawad.
In the west, the rebel-held town of Misrata is still reportedly coming under attack from pro-Gaddafi troops.
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."
Mr Hague urged others close to Col Gaddafi to "embrace the better future for Libya".
Since the uprising began in February, a number of senior Libyan officials have joined the opposition to Col Gaddafi.
They include the interior minister, the justice minister and the ambassadors to the US, the UN, France and India.
A Foreign Office spokesperson said on Wednesday: "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."
Mr Koussa arrived at Farnborough Airport in Hampshire on what is believed to have been a British military plane.
A Libyan spokesman denied that Mr Koussa had defected and said he was on a diplomatic mission.
Helping the rebels?
The CA's Nick Springate, who is near the frontline outside Brega, says the rebels' lead vehicle was destroyed by heavy artillery on Thursday. Other vehicles in the convoy have fled back towards Ajdabiya.Our correspondent says the lack of military leadership and hardware is stopping this determined force of unprofessional soldiers moving forward and actually taking on the pro-Gaddafi forces.
Maj Gen Suleiman Mahmoud, the second-in-command for the rebels, told the CA that rebels forces needed time, patience and help to organise themselves.
Meanwhile, US media reports say President Barack Obama has authorised covert support for the Libyan rebels.
The CIA and White House have both declined to comment on the reports.
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 took sole command of international air operations over Libya on Thursday. It said it had the means to enforce the UN resolution.
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.
Several thousand people have been killed and thousands wounded since the uprising against Col Gaddafi's rule began more than six weeks ago.
