Security sources in the country, the world’s biggest oil exporter, said that princes from the Royal Family had begun arriving in the capital, Riyadh, in the past few days, suggesting that the condition of King Fahd, believed to be 82, was “serious and worrying”.

A palace statement urged Saudis to pray for the king’s recovery. “We ask God to keep and care for the Custodian of the Two Holy Mosques and grant him health and well- being,” it said.

A Saudi source in Washington claimed last night that the king had already died.

King Fahd has been ruling in name only since he suffered a stroke in 1995, which left him with short-term memory loss and an inability to concentrate. Analysts were confident that his death would have no significant impact on the country as the succession has long been agreed.

Crown Prince Abdullah, a cautious reformer, has been the de facto ruler of the kingdom since then and is respected as a strong leader at home, where he has a secure power base, and abroad, where he is a personal friend of President Bush. In the event of King Fahd’s death, he is expected to take over as ruler while Prince Sultan, the Defence Minister, is expected to become the new Crown Prince.

“So there won’t be any change of policy or direction,” Gerald Butt, the editor of the respected oil newsletter Middle East Economic Survey, said. “Oil markets are likely to react but that’s irrational because there is no threat to supplies of Saudi oil.”

Crown Prince Abdullah, who was born in the early 1920s, already runs the day-to-day affairs of the country. By assuming the formal title of king his legitimacy may be enhanced, empowering him to carry out long-sought reforms. He will have to balance the conflicting aspirations of traditionalists and modernisers.

Al-Qaeda-linked activists in the kingdom, the birthplace of Osama bin Laden, have been under intense pressure from a determined security crackdown and a public relations offensive for several months and were unlikely to be able to capitalise on King Fahd’s failing health.

The kingdom came under a international pressure when it became apparent that 15 of the 19 suicide hijackers in the September 11 attacks in America in 2001 were Saudis, which put a strain on relations with Washington, its key Western ally.

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