The Foreign Office has advised against all travel to Benghazi since September, and on Thursday it said is aware of a "specific and imminent threat."
It urged all British nationals still in the eastern city of Benghazi to "leave immediately" and declined to comment on the nature of the threat.
The warning comes a day after U.S. Secretary of State Hillary Rodham Clinton testified to U.S. lawmakers about the handling of the deadly Sept. 11 attack on the American mission in Benghazi. The attack killed the U.S. ambassador to Libya and three other Americans.
In addition to the attack on the U.S. consulate, an Italian diplomat's car was fired on by militants
Britain's Foreign Office said it does not have a diplomatic presence in Benghazi, where the Libyan uprising against longtime dictator Muammar Qaddafi began in 2011.
Libya's security sharply deteriorated after Qaddafi's ouster and killing.
An expert on the North African region told CBS Radio News this week that the security situation hasn't improved in Libya since the deadly Benghazi attack.
"The problem with Libya is that there were no boots on the ground after the revolution, and the Libyan army and Libyan police basically evaporated," said William Lawrence, director of the North Africa Project at the Brussels-based International Crisis Group. "Basically in Libya, you've got 1,700 militias running the country still and not much of the police or army infrastructure."
Lawrence said the Libyan government isn't effectively finding solutions to build up the national army and the police force after the death of Muammar Qaddafi in late 2011.
"The new infrastructures needed to make Libya more secure, it just hasn't happened yet," said Lawrence. "One of the reasons is that all of these militias were formed to defend local neighborhoods, local villages, local towns, and what the government's asking them to do is to quit these militias and join the police force or join the army."