A new fleet of armoured limousines worth more than £2m is being deployed by Scotland Yard to protect Gordon Brown and key ministers from terrorist attack.

At least 10 high-security BMWs are providing David Miliband, the foreign secretary, John Hutton, the defence secretary, and the prime minister with protection from attack by Al Qaeda-linked extremists.

The mobile fortresses are equipped with security features designed to keep ministers alive in the face of an ambush by all but the most violent and audacious enemy.

They boast removable bullet-proof windscreens, in case the prime minister and his protection officers need to make an emergency exit, and an elaborate system to extinguish a fire under the chassis at the touch of a button.

The BMW 760s also have “run-flat tyres”, which allow them to be driven at speed for 30 miles — even if all four tyres have been shot out.

The custom-built “tank on wheels” weighs about three tons, but its 12-cylinder engine means it can accelerate to a top speed of 150 mph.

Brought in on the advice of senior officers in Scotland Yard’s specialist protection unit, the cars are replacing much of the fleet of British-built armoured Jaguars used to transport Tony Blair and the Prince of Wales after the September 11 attacks.

Two of the BMWs are understood to be permanently available to Brown. One is kept at Downing Street while the second is based in his Kirkcaldy and Cowdenbeath constituency in Fife.

A glossy manual handed to wealthy prospective buyers by BMW says the car’s armour plating can withstand a 33lb roadside bomb, while its roof can keep out two hand grenades exploding simultaneously.

Ballistics tests also show the armour can stop bullets from a Dragunov 7.62mm, a Russian-made high-velocity sniper’s rifle. The gun has a range of 1,300 yards and has been used in Iraq and by Al-Qaeda terrorists at training camps in Afghanistan.

An operating manual for the Dragunov was found when police arrested Samina Malik, the so-called “lyrical terrorist” who worked as a shop assistant at Heathrow airport. Her conviction for storing material likely to be of use to a terrorist was quashed at the Court of Appeal in June.

The BMW’s battery of anti-terrorist devices includes a remote ignition that allows the driver to start the engine from 150 yards away.

This feature enables passengers to check without risk whether explosive charges have been attached to the ignition system.

In the event of a chemical gas attack, three sensors located in the engine compartment signal any change in the air composition.

If a certain level of contamination is detected, the windows close automatically, the central locking system is engaged and fresh air vents are closed.

An automatic injection of oxygen provides occupants with breathable air for a short period, while concealed microphones allow them to talk to people outside the vehicle at checkpoints, for example, without having to open windows or unlock doors.

While the BMW 760 is now available for use by the cabinet, the royal family is understood to prefer more traditional vehicles. The Queen’s principal state car is a Bentley, while Prince Charles also supports British-built cars by using an armour-plated Jaguar.

Chauffeurs are also trained in “defensive” driving, which teaches them how to escape a terrorist ambush.

The BMWs, shipped from Germany, have been introduced following a security review in the wake of the 2005 London bombings.

MI5, the security service, says the threat of a terrorist attack by Al-Qaeda remains “severe” and that there are at least 2,000 extremists in Britain who could attempt such an attack.

A spokeswoman for Scotland Yard, which provides protection for ministers and funds part of the fleet, said she could not discuss security measures.

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