Saudi Arabia is teetering on the brink of collapse, fuelling Foreign Office fears of an extremist takeover of one of the West's key allies in the war on terror.

Anti-government demonstrations have swept the desert kingdom in the past months in protest at the pro-American stance of the de facto ruler, Prince Abdullah.

At the same time, Whitehall officials are concerned that Abdullah could face a palace coup from elements within the royal family sympathetic to al-Qaeda.

Saudi sources said the Pentagon had recently sponsored a secret conference to look at options if the royal family fell.

Demonstrations across the kingdom broke out in March, triggered by a fire in a girls' school in which 14 pupils died after the religious police stopped them escaping.

Unrest in the east of the country rapidly escalated into nationwide protests against the royal family that were brutally suppressed by the police. The Observer has obtained secret video footage of the protests smuggled out of the country last week that shows hundreds of Saudis, including women, demonstrating in support of the Palestinians and opposition to the regime.

The Foreign Office believes that the failure of Abdullah's recent Middle East peace plan could have terminally undermined his position.

The Crown Prince's main rival, Prince Sultan, the Defence Minister, has been vocal in his opposition to Abdullah's pro-Western policy. His brother Prince Naif, head of the Interior Ministry, has led a crackdown on the Saudi media in the wake of the demonstrations to stop any word of them leaking out.

Abdullah has even sent his own representative to Washington to counter the influence of the ambassador, Prince Bandar, a son of Prince Sultan.

Anti-Abdullah elements within the Saudi government are also thought to have colluded in a wave of bomb attacks on Western targets by Islamic terrorists.

The authorities have blamed the attacks on an alleged 'turf war' between Westerners involved in the bootleg alcohol trade and have jailed five Britons, a Canadian and a Belgian for the bombings. But British intelligence sources have confirmed that the attacks were carried out by Islamists linked to al-Qaeda.

Earlier this year, the accused men were handed sentences ranging from execution to long prison terms. But lawyers acting for the Britons have told The Observer that they could soon be free.

The tensions between the royal factions will intensify with the death of King Fahd. The condition of the king, in hospital in Switzerland, is 'unstable', doctors said.