All Britons have been urged by the Government to leave the Libyan city of Benghazi in response to a "specific and imminent threat" against westerners.
The Foreign Office has been advising against travel to most of the country since last September, but has now stepped up its warning.
A spokesman said: "We are now aware of a specific and imminent threat to westerners in Benghazi, and urge any British nationals who remain there against our advice to leave immediately.
"We have updated our travel advice to reflect this. The British Embassy in Tripoli has been in contact with British nationals for whom we have contact details to alert them to the advice."
The Foreign Office spokesman said: "We cannot comment further on the nature of the threat at this time."
The Government has been advising against all travel to much of Libya, and all but essential travel to Tripoli and a handful of other towns.
The dangers in the wake of Muammar Gaddafi's overthrow are said to include "indiscriminate" terrorist attacks against foreign travellers and kidnapping.
The French military action in Mali, which has received British logistical support, has also raised the threat of retaliatory strikes on westerners.
Benghazi was the stronghold of the Western-backed revolt that eventually ended Gaddafi's hold on power in Libya.
However, Britain has not had any diplomatic presence in the city since an attack on the US mission last September that killed American ambassador Chris Stevens and three of his colleagues.