Foreign Secretary William Hague seized on Kusa's defection as a way to urge other ministers in Libyan strongman's Moammar Gadhafi's government to abandon the "crumbling" regime in favor of a "better future" for the country, The Guardian reported.
Hague renewed his call for Gadhafi to step down, saying the loss of one of his closest allies indicated the leader's regime was "fragmented, under pressure and crumbling from within."
"Gadhafi must be asking himself who will be the next to abandon him," Hague said. "We reiterate our call to Gadhafi to go."
Scottish officials say they want to interview Kusa about the 1998 bombing of Pan Am Flight 103 over Lockerbie, Scotland, in which 270 people were killed.
White House press secretary Jay Carney said Thursday the Obama administration was "consulting with the British" on the Kusa defection and the Lockerbie matter.
Kusa was a leading member of the Libyan Bureau for External Security, called the Mathaba, The Scotsman reported. The bureau has been linked to the Lockerbie bombing.
Libyan agent Abdelbaset Ali Mohmed al-Megrahi was the only person convicted in the bombing, but he was sent back to Libya by Scotland in August 2009 on purported humanitarian grounds. He supposedly had only three months to live but continues to reside in Libya.
A Scottish official spokesman said, "We have notified the Foreign and Commonwealth Office that the Scottish prosecuting and investigating authorities wish to interview Mr. Kusa in connection with the Lockerbie bombing. The investigation into the Lockerbie bombing remains open and we will pursue all relevant lines of inquiry."
Hague said Kusa, Libya's No. 2 minister behind Prime Minister Baghdadi al-Mahmoudi, fled to Britain Wednesday on a flight from Tunisia arranged by British intelligence services. He said he was "no longer willing" to represent Gadhafi's regime.
After arriving in London, Kusa went into talks with high-ranking British Foreign Office officials, where he resigned his post as Libya's foreign minister, the Foreign Office said.
"He traveled here under his own free will," a Foreign Office statement said. "His role was to represent the regime internationally -- something that he is no longer willing to do."
Kusa's defection will provide a source of invaluable intelligence in terms of understanding the situation within the Libyan leader's inner circle, officials said. But it also could lead to questioning about his possible involvement in, or knowledge of, atrocities including the downing of Pan Am Flight 103.
It was not clear who, if anyone, would succeed Kusa in the Libyan government, a European official told The Wall Street Journal.
Kusa is one of a growing number of senior Libyan officials who reached out to at least 12 countries to discuss either defections or ways of ending the Libyan conflict, U.S. officials told the Journal.
The Obama administration hopes a larger rebellion will take place as these senior officials leave, the newspaper said.
"This is a very significant defection and an indication that people around Gadhafi think the writing's on the wall," a senior U.S. official said.
Other top defections since the uprisings began Feb. 15 include Justice Minister Mustafa Abdul-Jalil Feb. 21 and Interior Minister Abdul Fatah Younis Feb. 22.
Kusa was Gadhafi's spy chief from 1994 to 2009, a period when Libya was suspected of being behind a string of terrorist attacks in Europe that killed Libyan dissidents and foreigners.
As foreign minister, Kusa appears to have been key in Libya's decision to abandon its weapons-of-mass-destruction programs, which led to the lifting of international sanctions against Libya, diplomats told the Journal.
He also was central to reaching deals to compensate the victims of the Lockerbie bombing and the bombing of a French airliner the following year, the newspaper said.
Kusa had referred to Benghazi rebel leaders as criminals and al-Qaida supporters, but "was notably uncomfortable in making public statements on behalf of the regime in recent weeks," particularly when he had to present "rational" explanations for Gadhafi's "rambling statements on national television," The Daily Telegraph said.