The Foreign Office is urging Britons to leave the Libyan city of Benghazi immediately "in reponse to a specific threat to Westerners".
It said that the British Embassy in Tripoli has been in contact with British Nationals whose details it held.
But although it could not comment on the nature of the threat, the website said that it was aware of a "specific and imminent".
It stated: "We advise against all travel to Benghazi and urge any British nationals who are there against our advice to leave immediately."
