The Foreign Office is working with the Libyan government in response to a "potential threat" against the British Embassy in Tripoli.
Travel advice remains unchanged, with officials warning against all but essential travel to the Libyan capital.
A Foreign Office spokeswoman said: "We are aware of reports of a potential threat against the British Embassy in Tripoli and we are liaising closely with the Libyan government.
"There is no change to our travel advice: we still recommend against all but essential travel to Tripoli."
The latest alert comes days after Britons were urged by the Government to leave the Libyan city of Benghazi after it became aware of a "specific and imminent threat".
Britons have also been warned of a "specific threat" against Westerners in Somaliland.
UK nationals have been advised for some time to stay away from the breakaway territory because of the high threat of criminal and terrorist kidnappings.
The Foreign Office yesterday warned any Britons who remained in the country to leave immediately.
The danger posed by terrorists in Africa has been highlighted by the attack on the In Amenas gas plant in Algeria, which is believed to have claimed the lives of six Britons and one UK resident.
French military action against Islamist extremists in Mali, which has been supported by the British, has raised the threat of reprisals against Westerners.