MI5 has drawn up a secret list of more than 350 key British institutions considered potential terrorist targets in the wake of the threat posed by al-Qaeda forces.

Key government buildings and installations vital to the economy are on the "critical national infrastructure" list.

All the sites have had their security profiles reassessed with the aim of maintaining basic national services in any attack, The Times reports.

The move comes in the wake of the 11 September attacks on America, for which Osama Bin Laden and his al-Qaeda network are held responsible.

If attacked, Government in underground bunkers

The Houses of Parliament are considered to be among the places at risk, the newspaper reports.

In the event of any such an attack, there are plans in place to move the government and civil service out of the city into underground bunkers to maintain a skeletal administration.

BT's Telecom Tower is seen as a 'key military point' and the military would seize control of its protection in an attack to maintain communications.

Tight security

Oil refineries and vital communication installations are among the centres which the newspaper reports have been visited by MI5 agents working to step up security there.

The list is thought to include the country's 15 nuclear power stations, the main National Grid sites, oil installations and petrochemical facilities.

Tighter security measures have also been stepped up at defence companies and research centres, such as the chemical defence agency at Porton Down in Wiltshire.

BT Telecom Tower 'central' to communications

The atomic weapons establishment at Aldermaston in Berkshire has also been checked, it is believed.

Military barracks, such as the nuclear submarine bases at Faslane on the Clyde, are seen as potential terrorist targets despite armed protection.

But some key centres such as London's Canary Wharf Tower and Birmingham's Spaghetti junction, seen as potential targets, are reported to not receive special protection.

They are not considered essential to the running of Britain.

The threat of suicide bombers makes these institutions virtually impossible to insure.

Centres listed

Nuclear power stations National Grid sites Houses of Parliament BT Telecom Tower Atomic weapons establishment

In the wake of Britain's support for America in its war on terrorism and as one of its oldest allies, security chiefs believe the country faces an increased risk of attack.

Dealing with this is down to MI5, which is responsible for giving security and protective advice to these institutions and installations.

A Home Office spokeswoman said: "It is government policy not to comment on operational security matters.

"All security measures are kept under constant review and have been before and after the events of September 11."