New barricade to shield No10

Security measures outside Downing Street are to be dramatically increased.

Work has begun on a new barricade outside the gates to prevent attacks from suicide bombers.

Posts 4 feet tall are being installed as a permanent measure.

Downing Street - home to both the Prime Minister and Chancellor Gordon Brown - is seen by terrorists as a symbolic target.

No10 has as been earmarked by the intelligence services as a potential target for al Qaeda bombers seeking revenge for Britain's involvement in Iraq and Afghanistan.

One of al Qaeda's preferred methods of attack is a lorry packed with explosives, but the hydraulically operated posts will prevent vehicles from ramming the gates to get access.

A Scotland Yard spokesman said: "We are updating the security measures at Downing Street to ensure we have the best equipment.

"This is part of an ongoing security review. The concrete posts will replace temporary barriers that had been erected outside the gates."

The latest measures come after an additional set of gates was installed in Downing Street 18 months ago. Access to the rear of No10 has now been

sealed off by a security cage and a permanent armed patrol.

Earlier this month, a man launched a sledgehammer attack on Downing Street.

The street's original gates were erected in 1989 after the bombing of the Inglis barracks, Mill Hill, in

August 1988. But they proved no obstacle to the IRA in 1991, when it launched a mortar attack on John Major's government at the height of the Gulf War.

The bombs were fired from a converted van in Whitehall, forcing the Prime Minister, his war cabinet and his advisers to duck under the table as windows shattered. Although the assault failed to kill or injure, it was widely regarded as a propaganda victory for republicanism and an embarrassing failure for British security.

Early this year, anti-war demonstrators threw white paint at the gates.