The Foreign and Commonwealth Office believes terrorists are in the final stages of planning attacks in the capital Mogadishu.
Officials have warned against all travel to every part of the war-torn country.
They advised all expat Brits to leave, adding there is a specific threat to Westerners in the autonomous region of Somaliland.
A spokesman for the FCO said: “We have amended our travel advice for Somalia.
“Our advice makes clear that there continues to be a big threat from terrorism and that the FCO believes that terrorists are in the final stages of planning attacks in Mogadishu.”
The high threat from terrorism includes kidnapping. There is no British representation in any part of Somalia and the FCO said it is unable to provide consular assistance there.
