LONDON (AFP) - Britain has issued the highest possible security alert to British shipping companies operating off the coast of Yemen, an expert said on Tuesday, amid a worldwide terror alert focused on the country.

Mr Neil Roberts, a senior insurance underwriter at Lloyd's Market Association, said the Department for Transport had issued a "level three" warning about Yemen on Monday.

Under the International Ship and Port Facility Security (ISPS) code, level three is "exceptional", suggesting a "probable or imminent risk of a security incident".

Insurers have listed Yemen as a high-risk area for at least the last five years, requiring shipping companies to alert their underwriters before they head to the country.

But Mr Roberts said that the government advice suggested a more pressing threat.

"They have put it at security level three, which is unusual," he told AFP.

"This is a wake-up call for British shipowners that the risk has increased significantly."

The Department of Transport refused to confirm the threat level, saying only that it was kept under "constant review".

A spokesman said: "The department applies a system of worldwide security levels for UK flagged shipping in accordance with the ISPS code.

"These security levels take account of concerns such as the current situation in the Yemen. Companies operating vessels under a UK flag are informed promptly of any changes to the security levels,'' she said.

"The levels in place, and the security measures required to be implemented at each level, are kept under constant review."

Britain said on Tuesday that it had withdrawn all staff from its embassy in Yemen, after the United States ordered its citizens to leave the country following concerns of an Al-Qaeda attack.