Britain upgraded its advice for travel to Saudi Arabia on Sunday to say that terror attacks it has warned of in the oil-rich Gulf state could be in the final stages of preparation.

In advice about foreign travel published on its web site, Britain's Foreign Office has warned British nationals for several months against all but essential travel to the kingdom, especially after attacks in Riyadh in May and November.

It has repeatedly said it thought more attacks were planned in Saudi Arabia. However, on Sunday it added that "these could be in the final stages of preparation".

A Foreign Office spokesman would not comment on whether the change was due to new information. "Our travel advice is based on all the information available to us at any one time," he said.

The new guidance coincided with news that sky marshals will travel on some British passenger planes following warnings from the United States about a heightened terror threat.

It also followed a British newspaper report on Sunday which said Saudi security forces had seized light planes packed with explosives, foiling a plot to blow up a Western airliner.

The report quoted Patrick Mercer, security policy chief for Britain's opposition Conservative party. Saudi Arabia denied the report.

(Reuters)