Saudi Arabia has been strengthening its border defences since the crisis in Iraq began

Saudi Arabia has deployed thousands of troops from Egypt and Pakistan along its frontier with Iraq, amid fears of invasion by the al-Qaeda splinter group that has declared a radical Islamic state across the border.

Panicked by the advance of the Islamic State of Iraq and al-Sham (Isis), Riyadh has taken the drastic step of calling in military assistance from its close allies to shore up the porous 500-mile border, Gulf security sources said.

Saudi Arabia spent an estimated £35 billion on defence last year, leapfrogging Britain as the world’s fourth-largest military spender. This massive outlay highlights Riyadh’s unease about whether its defences would hold if the jihadists launched a direct attack.

Uppermost among Saudi concerns is the defence of Mecca. Members of Isis have