Mohammed Ismail, an aide to one of Col Gaddafi's sons, was apparently told the Libyan leader had to give up power. The Foreign Office said "in any contact that we do have we make it clear that Gaddafi has to go". Meanwhile, the US says air attacks have destroyed about a quarter of the Libyan government's military's capabilities. Following the defection on Wednesday of the Libyan Foreign Minister, Moussa Koussa, there were unconfirmed reports that other senior Libyans were defecting. Al-Jazeera said the intelligence minister, deputy foreign minister and General People's Congress speaker were awaiting flights in Tunisia.
Oil Minister Shukri Ghanim has denied any intention to leave Libya. Ali Abdul Salaam Treki, who was recently named as Libya's permanent representative to the United Nations, is not accepting the post.
A government spokesman told reporters that they were all in Libya, but that it was also possible that some had left the country on a mission.
Koussa debriefed
BBC diplomatic correspondent James Robbins understands that Mr Ismail - a senior aide to Saif al-Islam Gaddafi - visited London in recent days.
The Foreign Office refused to confirm the visit, saying it was "not going to provide a running commentary on our contacts with Libyan officials, but that in any contact that we do have we make it clear that Gaddafi has to go".
It seems that the envoy may have wanted to explore a possible exit strategy for the Libyan leader, but the Foreign Office insist their policy is to encourage all those around him to abandon what they call "a brutal regime" and embrace a better future for Libya, our correspondent says.
It is not clear on whose authority within the regime Mr Ismail was in London, but he is thought to have returned to the Libyan capital, Tripoli, he adds.
There has been mounting speculation that Col Gaddafi's sons, particularly Saif al-Islam, Saadi and Mutassim, are willing to discuss exit strategies, either for the whole family or only the Libyan leader.
The reports of Mr Ismail's visit come as British diplomats and intelligence officials continue to debrief Moussa Koussa, Libya's foreign minister and former long-time head of its external intelligence service.
UK Prime Minister David Cameron said Mr Koussa's defection told "a compelling story of the desperation and the fear at the very top of the crumbling and rotten Gaddafi regime".
He also insisted that Mr Koussa had not been offered immunity from prosecution. Scottish prosecutors have asked to interview him about the 1988 Lockerbie bombing, which left 270 people dead, though government sources told the Guardian they did not think he was involved.
Rebels 'outnumbered'
Meanwhile, the Chairman of the US Joint Chiefs of Staff, Adm Mike Mullen, said coalition air attacks had destroyed about a quarter of the Libyan government's military's capabilities.
But Adm Mullen said this did not mean that Col Gaddafi's forces were close to breaking point; he said they still outnumbered Libyan rebels about 10-to-one.
"We have actually fairly seriously degraded his military capabilities. That does not mean he's about to break, from a military standpoint, because that's not the case," he told the US House of Representatives armed services committee.
He also revealed that bad weather had stopped the coalition from identifying targets over the past three or four days.
The rebels in Libya have been struggling to regain the initiative after several days of losing ground to Col Gaddafi's forces.
They are trying once more to advance westwards along the Mediterranean coast.
But they were halted by the government troops' superior firepower while attempting to reoccupy the oil town of Brega, a town about 800km (500 miles) east of Tripoli. Gaddafi loyalists earlier overran Ras Lanuf.
Further west, pro-Gaddafi forces continued to pound the besieged city of Misrata with artillery and tank fire.
"If it continues for a couple more days, I'm afraid it will overwhelm the city and it will be a complete massacre," a resident told the BBC.
Nato has meanwhile said it is investigating reports of civilian casualties caused by air strikes on Tripoli. Earlier, the top Vatican official in the Libyan capital, citing witnesses, said 40 civilians had been killed.
