A British Foreign Office statement said that Koussa "has told us that he is resigning his post."
The Foreign Office said Koussa flew into an airport near the capital on March 30. He has subsequently spent hours talking to British officials.
He arrived in Britain after a two-day stay in Tunisia, which Tripoli had officially described as a "private visit."
Several senior members of Qaddafi's entourage have defected since the uprising against his 42-year-rule began more than a month ago.
Washington quickly hailed Koussa's departure as a major blow to the Qaddafi regime.
Koussa's defection comes as rebels in Libya are retreating from former strongholds along the eastern coast after advances by forces loyal to Qaddafi, as world powers consider arming outgunned rebel forces.
Loyalists supported by tanks and artillery reached the outskirts of Ajdabiya, some 160 kilometers east of the rebels' main stronghold of Benghazi, after retaking the oil hubs of Ras Lanuf, Uqayla, and Brega.
Qaddafi's latest counteroffensive came after rebel forces on March 29 had advanced to within 100 kilometers of Sirte, aided by coalition air strikes.
In the west, loyalists were still besieging the rebel-held town of Misurata.
The United States and Britain say they have not ruled out arming the rebels.
Meanwhile, reports say the CIA sent operatives to Libya earlier this month after the agency's station in Tripoli was forced to close.
compiled from agency reports