It is not known how long the embassy in Yemen will be closed

A statement on the FO's website said the embassy was closed in response to "specific security concerns".

The state in the south of the Arabian peninsula has seen a number of terrorist attacks in recent years.

Reuters reports that specific information had been received that a terror attack was "in the final stages of planning".

The Foreign Office website said: "We are concerned that terrorist attacks in Saudi Arabia during 2004 could inspire extremists located in neighbouring countries to carry out similar attacks."

The statement did not say how long the embassy would stay closed. It warned Britons to be extra careful in places used by foreigners such as hotels.

British diplomats were not available to comment.

A Yemeni government official said the closure was a "routine procedure" by some embassies in order to stay on alert for any possible attacks.

He told Reuters: "There are no serious threats. Yemeni security forces are ready to face any emergency and we were not informed of any specific threats."

In 2003 Yemen said it had foiled planned attacks on U.S. and British embassies in the capital Sanaa.

The impoverished country, the ancestral home of al-Qaeda leader Osama bin Laden, has co-operated closely with the US-led "war on terror" and has arrested hundreds of al-Qaeda suspects since the 11 September 2001 attacks.