Moussa Koussa, the former Libyan foreign minister who resigned his position and fled to the UK, has not been offered immunity from prosecution and is "voluntarily talking" to authorities, William Hague, Britain's foreign minister, has said.
Koussa was staying in a safe and secure place and engaged in ongoing discussions with British diplomats, including some who worked at the now-shuttered embassy in Libya, Hague said.
"His [Koussa's] resignation shows that [Muammar] Gaddafi's regime ... is fragmented, under pressure and crumbling from within," he said.
Hague said Koussa had been his contact with the regime in recent weeks and that he had spoken with him several times.
"One thing I gathered between the lines of my telephone calls ... was that he was very distressed and dissatisfied" by the regime's response to protests, Hague said.
A Libyan government spokesman confirmed on Thursday that Koussa had resigned but said that Gaddafi still enjoyed the support of his people.
Moussa Ibrahim said that Koussa's decision was personal and "other people will step in and do the job".
Ibrahim says Koussa had been given permission to go to Tunisia because he was sick with diabetes and high blood pressure.
He said the goverment did not know he would go to London.
On Thursday, a second top official said he would not serve in Gaddfai's regime.
Ali Abdessalam Treki, a former foreign minister and UN general assembly president, had been named to represent Libya at the UN after a wave of defections early in the uprising.
Treki, who is currently in Cairo, said in a statement posted on several opposition websites that he was
not going to accept that job or any other.
"We should not let our country fall into an unknown fate," he said. "It is our nation's right to live in freedom, democracy and a good life."
Scottish authorities said on Thursday that they wished to interview Koussa over the 1988 Lockerbie bombing.
"We have notified the Foreign and Commonwealth Office that the Scottish prosecuting and investigating authorities wish to interview Mr Koussa in connection with the Lockerbie bombing," Scotland's crown office said in a statement.
"The investigation into the Lockerbie bombing remains open and we will pursue all relevant lines of inquiry."
The bombing over the Scottish town in 1988 killed 259 people, mostly Americans, on the plane and 11 on the ground.
Last month, Mustafa Abdel Jalil, the former Libyan justice minister and leader of the rebel's interim national council, said that Gaddafi ordered intelligence officers, including the convicted bomber Abdel Baset al Megrahi, to carry out the bombing.
Jim Swire, whose daughter Flora was among those killed, said Koussa's arrival in Britain was an opportunity to finally shed some light on the bombing.
"Koussa was at the centre of Gaddafi's inner circle. This is a guy who knows everything," he said.
"I think this is a fantastic day for those who seek the truth about Lockerbie. He could tell us everything the Gaddafi regime knows."
Koussa's resignation deals a serious political blow to Gaddafi, Libya's leader for 41 years, even as the regime's troops beat back rebels in the east.
Fighting reached Brega, a village near a large petrochemical plant and oil exporting facility, on Thursday, after shelling from mortars and Grad rockets forced the disorganised opposition fighters to pull back from Ras Lanuf the day before.
Despite the lost ground, the opposition could savour some positive developments on Thursday. Koussa, described as being a pillar of Gaddafi's regime and a member of his inner circle, became the highest-profile member of the government to resign.
Western media also reported that rebels had begun to receive support from British and US special forces troops.
Agents from the US Central Intelligence Agency and British Special Air Service have been working on the ground in Libya for weeks, helping to identify Gaddafi's troops for air strikes and meeting with rebel forces, reports said, possibly to pave the way for training and arms shipments.
Observers have speculated that Koussa, a longtime government employee who led the regime's intelligence agency for five years, could now be providing sensitive information as well.
What arrangement he will make with the British government, considering his possible exposure to lawsuits or criminal prosecution for being involved with the regime, remains unclear, and Hague said Koussa has not been granted immunity from prosecution in the UK or elsewhere.
Koussa was expelled from the UK, where he served as ambassador for less than a year, after saying the regime planned to kill two Libyan dissidents.
The international community, sensing a hard road ahead in their effort to assist the rebels' faltering military campaign against Gaddafi, want to see more leading regime figures abandon Libya.
"It is not through actions of war that we can make Gaddafi leave, but rather through strong international pressure to encourage defections by people close to him," Italian foreign minister Franco Frattini told Canale 5 television.
Hague said the British government encouraged others around Gaddafi to abandon him and "embrace a better future for Libya."
Many other Libyan government figures have resigned since the uprising against Gaddafi began on February 15 - interior minister Abdel Fattah Younis and justice minister Mustafa Mohamed al-Jalil have both left, as have numerous ambassadors around the world.
But the north African nation's future still lies in doubt, as Gaddafi's superior military continues to defeat lightly armed and undisciplined rebels, even with a coalition of Western and Arab forces using the recently passed UN Security Council resolution as a mandate to crush the regime's army, air force and navy.
Rebels told reporters on Thursday that Gaddafi's troops were driving through Brega, firing indiscriminately, and that some fighters had already withdrawn to Ajdabiya, a crossroads town just 140km from Benghazi, the center of the uprising.
Though the international coalition, which includes France, the UK the US and Qatar, has made it clear they will not allow Gaddafi's troops and armoured columns to overrun Benghazi, it appears that their no-fly and "no-drive" zone has ensured a stalemate, and nothing more.
Some countries, including the US and the UK, have said they are considering supplying the rebels with weapons, sparking a debate between those who think the coalition needs to go further and make it a goal to kill or capture Gaddafi himself and others who fear getting embroiled in what they see as a conflict that could turn into tribal-tinged civil war.
Sheikh Hamad bin Khalifa al Thani, the emir of Qatar, said on Thursday that Libyans must decide their own future and that the coalition campaign in the country is limited to protecting civilians and "nothing else," suggesting he would not support the goal of removing Gaddafi by force.
But Qatar has come out more forcefully on the rebels' behalf than most other nations: It has sent fighter jets, helped set up a new Libyan television station in Doha, the capital, and is only the second country to recognise the opposition Transitional National Council as the sole representative of the Libyan people.
Qatar has also reportedly agreed to give weapons to the rebels and market oil from the opposition-held east.