Osama bin Laden: Still active?

Britain's embassy in Saudi Arabia has been shut down in response to a heightened al Qaeda terror threat.

The move comes amid growing concerns over security at British and American installations in the Middle East.

The United States is also on a terror alert after intelligence revealed a possible al Qaeda attack.

Washington's Department of Homeland Security raised the state of risk to "level orange" or "high alert", one category down from "severe".

Intelligence information in the wake of the attacks in the Saudi capital, Riyadh, last week suggested the terrorist network behind September 11 was gearing up for more attacks.

Britain, the United States and Germany have closed their Riyadh embassies and updated their travel advice for the region.

The Foreign Office said that attacks "could involve the use of chemical and biological materials" and advised against "all but essential travel".

With America having failed to capture al Qaeda leader Osama bin Laden, his deputy Al Zawahiri and the Taliban leader Mullah Omar, the security services remain concerned at the ability of the groups to strike at American or US-associated targets."As long as these figureheads are alive, recruits will flock to the cause," a British official said.

Whitehall sources said that Britain's decision was taken independently and was not merely a response to America's. "We assess that the threat is live," said one official.

The move also came after a warning by Prince Bandar bin Sultan, the Saudi ambassador to Washington, who said on Monday that he feared another "big" attack in the US or Saudi Arabia was imminent.

However one Washington intelligence source was reported as telling the Guardian newspaper that there was "no specific intelligence" to justify an alert, which he described as "pure politics".

The Home Office has so far issued no new alert affecting domestic security in Britain.