The Foreign Office has urged Britons to leave the Libyan city of Benghazi in response to a "specific and imminent threat" against Westerners.
The FCO has been advising against travel to most of the country since last September, but has now stepped up its warning.
Germany has also urged its citizens to leave Libya's second city, citing knowledge of a specific threat.
The Dutch government has warned its nationals that "staying in this area is not to be advised".
Air Malta cancelled Thursday's flights to Benghazi following the British advisory, adding that "the airline will review the situation on an ongoing basis".
A Foreign Office 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."
Sky News foreign affairs editor Tim Marshall said: "As I understand it there are somewhere between one and two dozen Britons in Benghazi.
"This is not to do with the oil fields. This is to do with the city of Benghazi and its immediate surroundings itself.
"I believe all of the Britons have been told of this credible threat to them. Benghazi airport is working, so they have a choice to go or not."
Libya has responded to the FCO's warning, saying the call for Britons to leave Libya's second city was not justified.
"Nothing justifies this reaction," Libya's Deputy Interior Minister Abdullah Massoud said.
"There are question marks about this communique," he added.
He said Tripoli would demand an explanation from London over the remarks.
The threat against Westerners comes one day after US Secretary of State Hillary Clinton responded fiercely to a review of security at US diplomatic missions.
The review followed an attack on the US mission in Benghazi in September last year which killed Ambassador Chris Stevens and three other Americans.
In her last formal congressional testimony on Capitol Hill as America's top diplomat, Mrs Clinton took full responsibility for the department's mistakes leading up to the attack.
"Nobody is more committed to getting this right. I am determined to leave the State Department and our country safer, stronger, and more secure," she said.