A "specific threat" has been identified to westerners in Somaliland, the Foreign Office said in a warning to all Britons who have ignored previous warnings they should flee immediately.
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.
A number of individuals with dual nationality are thought to travel there regularly, however, and some aid agencies are also believed to have staff in the country.
"We are now aware of a specific threat to Westerners in Somaliland, and urge any British nationals who remain there against our advice to leave immediately," the FCO said.
"As our travel advice continues to make clear, kidnapping for financial or political gain, motivated by criminality or terrorism, remains a threat throughout Somalia."
It comes just 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".
The terror threat in Africa was thrust into the spotlight by the Algerian gas field hostage crisis which is believed to have claimed the lives of six Britons and one UK resident.
French military action in Mali, which has received British logistical support, has raised the threat of retaliatory strikes on westerners.