Speaking publicly for the first time since coming to the UK, Moussa Koussa told the BBC that the unity of Libya was essential to any settlement.
His comments came after rebels rejected an African Union ceasefire proposal.
The AU says Col Muammar Gaddafi has accepted the plan, but on Monday his forces attacked the city of Misrata.
After eight weeks of fighting, pro-Gaddafi forces have recently pushed rebels back towards the east along Libya's northern coast, but Nato has thwarted their advance.
Mr Koussa was Col Muammar Gaddafi's foreign minister until 12 days ago, when he fled to London.
BBC's security correspondent Gordon Corera said he was told Mr Koussa was not ready to be interviewed, but would give a prepared statement.
"I ask everybody to avoid taking Libya into civil war," Mr Koussa said. "This would lead to so much blood and Libya would be a new Somalia."
"More than that, we refuse to divide Libya. The unity of Libya is essential to any solution and settlement for Libya."
Libya's Minister for Social Affairs, Ibrahim Zarouk al-Sharif, said he could not comment on Mr Koussa's statement while the former foreign minister was "captured" in a hostile country.'Sacrifice'
Mr Koussa is a former head of Libyan intelligence and has been accused of being involved in the 1988 Lockerbie bombing.Continue reading the main story Gordon Corera Security correspondent, BBC News
The most high-profile defector from the Libyan regime made his statement to the BBC in Arabic at a location in central London.
There was tight security but the words Moussa Koussa spoke appeared to be his own and no British officials were present in the room.
The former foreign minister said he had served Colonel Gaddafi for 30 years but as the security situation deteriorated he decided to leave. He said he knew this would cause him problems.
He said he had good relations with the UK and had worked closely with people in the country to deal with terrorism and weapons of mass destruction.
The former intelligence chief was interviewed by Scottish police last week but he made no reference in his statement to questions about his past and any knowledge or involvement in the 1988 Lockerbie bombing.He has been staying at an undisclosed location since travelling to the UK from Tunisia.
In his statement on Monday he said he had been "devoted" to his work for 30 years under Col Gaddafi, and was confident that it was serving the Libyan people.
However, he said, after recent events "things changed and I couldn't continue".
"I know that what I did to resign will cause me problems, but I'm ready to make that sacrifice for the sake of my country," he said.
He added that the solution in Libya would come from the Libyans themselves, through discussion and democratic dialogue.
The UK and its allies have a responsibility to ease the dialogue so that Libyans can build a democratic country, he said.
The BBC's Middle East editor Jeremy Bowen said Mr Koussa's decision to speak in Arabic suggested he wanted to send a message back home - to both sides.'Escalation'
Earlier, Libyan rebels based in the eastern city of Benghazi said they were rejecting the AU-proposed truce because it did not include plans for Col Gaddafi's departure from power.
The US, the UK and Italy have repeatedly called for the Libyan leader to step down.
As well as an immediate ceasefire, the AU deal proposed talks between the government and rebels, the unhindered delivery of humanitarian aid, and the suspension of Nato airstrikes.
But Nato - whose air strikes have been targeting pro-Gaddafi forces - has said it will continue to fly missions over Libya and take action when it sees a threat to civilians.
It says any ceasefire must be credible and verifiable.
A resident in the besieged western city of Misrata told the BBC that Col Gaddafi's forces had been firing rockets there from early on Monday.
Several people were reportedly killed by the government bombardment, and a rebel spokesman said fighting in the city had intensified.
"This is a new escalation and a new level," rebel spokesman Mahmoud Amloda told Reuters news agency. "We do not see any sign of a ceasefire."
Libyan authorities have prevented journalists from reporting freely from Misrata, and the accounts could not immediately be verified.
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