From the shore it looked like just another oil tanker manoeuvring into position at offshore platform number four at the Ras Tanura loading terminal on Saudi Arabia’s eastern coast.

On a normal day, more than 4m barrels of prime Gulf crude are pumped into ships waiting at the sprawling Sea Island facility to distribute Saudi oil to the world. Local workers barely noticed as the tanker’s hull loomed closer.

Fifty miles to the southwest, near Abqaiq in the foothills of the al Aramah mountains, a young bearded Saudi who had trained in Afghanistan and fought in Iraq loaded a shell into his Russian-built 120mm mortar.

A few hundred feet below him sat pump station number one, the first crucial link in the oil pipeline linking Abqaiq’s refineries — the world’s largest oil-processing complex — to the Red Sea terminals on the other side of the country. The Saudi gunner looked at his watch. The explosions would be simultaneous.

At precisely the moment the commandeered tanker packed with ammonium nitrate demolished the Ras Tanura platform, a well-aimed barrage of mortar shells smashed into the pumps and valves that regulate the westward flow of oil. A tower of burning oil sprayed high above the desert as a rupture in the pipeline spread.

For more than 30 years American planners have been dreaming up worst-case scenarios like this one, first described last year in a book by Robert Baer, a former CIA operative specialising in the Middle East.

For much of that time, the house of Saud shrugged off the threat and doomsday fears eased. Yet last week’s attack on the Oasis Residential Resorts, a luxurious housing compound a few miles up the Gulf coast at al-Khobar, raised questions about oilfield security and Al-Qaeda’s strength in the country where Osama Bin Laden was born.

The murders of 22 foreigners — one of them a Briton whose body was tied to a car and dragged through the streets by his attackers — provoked an instant spike in oil prices and forced western governments to re-examine their contingency plans. The prospect of a catastrophic interruption of oil supplies from the world’s largest producer is once again haunting the West.

Not only does Saudi Arabia control a quarter of the world’s proven oil reserves while pumping more than 8m barrels a day, it is the only main producer with enough spare capacity to neutralise price hikes caused by man-made or natural disasters.

In the past this ability to increase the flow of oil to international markets by up to 2m barrels a day has proved vital to the smooth running of the world economy, allowing it to calm violent swings in supply and demand. It was used in the world oil crisis of 1974 to break the Opec embargo against the West, for example, and again on September 12, 2001, less than 24 hours after the attacks on the World Trade Center and Pentagon.

“After 9/11 the United States experienced only a slight inflation spike in the wake of the most devastating terrorist attack in history,” said Baer. “Had the same surplus capacity been taken out of play with 20lb of Semtex, all bets would have been off.”

The recent wave of violence against foreigners in Saudi Arabia has fanned international concern about the stability of the ruling royals and the threat they now face from Islamic militants sympathetic to Al-Qaeda. It has already forced Opec into remedial action: oil ministers agreed in Beirut on Thursday to raise output by 8.5% in the hope of calming markets and reducing the so-called “fear premium” that is pushing up the price of crude. Saudi oil production is now operating at full capacity.

“The Saudi royal family feels threatened,” Baer said. “The question is whether they can control the chaos.”

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