MANY of London’s most popular tourist landmarks are to be surrounded by concrete after a CIA-MI5 summit called for special protection zones to guard against a “spectacular” al-Qaeda terrorist attack in Britain.

Scotland Yard has also armed elite undercover squads trailing suspected Islamic militants and told them to shoot to kill if they believe a suicide attack is imminent.

The concrete barrier put up around the Houses of Parliament to prevent a suicide strike by lorry bombers will soon be replicated at other landmark buildings after Eliza Manningham-Buller, the head of MI5, and David Blunkett reviewed vulnerable targets in the light of her meeting with the CIA Director George Tenet. Security has also been reinforced at the entrance to Downing Street and Horse Guards Parade has been sealed off to tourists.

A senior Whitehall source told The Times last night: “All picture-postcard sites in London are clearly something that could be of interest to terrorists. While there is no imminent threat we know of, the belief is that it is only a matter of time before something happens here.”

Police and political leaders emphasised last night that they had uncovered no specific threats against the capital, but the wave of bombings in Morocco and Saudi Arabia that left 75 dead and the threatening message from a leading al-Qaeda figure this week prompted an urgent reassessment of security in the capital. Britain is on the second- highest alert; it would move to the top alert if there were specific intelligence about a time, place and target.

The security concerns have prompted Scotland Yard to arm for the first time undercover teams whose role is to identify and trail suspected Islamic terror squads. These officers have been told to shoot to kill if they believe a suspect is about about to trigger a suicide attack.

An extra 150 police officers were also patrolling London streets yesterday, more armed patrols were ordered into all major airports, and security was stepped up at the Channel Tunnel and ports for the start of the Bank Holiday weekend.

A series of rolling road-blocks, manned by armed police, were set up in the City of London yesterday with the most obvious sign the overnight installation of a 300ft wall of reinforced concrete blocks outside Parliament.

The slabs are 4ft high, 2ft thick and weigh about 2½ tons each, and a double layer was installed at “the most sensitive points” including police posts and the entrances facing Parliament Square.

For the moment Parliament — which is also guarded from the Thames by a special police unit — remains open to the public, although the authorities are reviewing access to London’s main attractions.

Scotland Yard said that the barriers would remain “for the foreseeable future”, but would not say which other buildings might be “hardened” against terrorist attacks.

Ken Livingstone, the Mayor, said that the ring of concrete was “a sign of the times we live in”. He said that people should be vigilant, but urged Londoners and tourists not to be put off going out and about and enjoying the capital.

This type of heightened protection is already place in many American and European cities. In Washington, there is a perimeter barrier and batteries of Patriot missiles around the White House. In New York, troops are guarding some subway stations and police have been given chemical and biological protection suits.

While security arrangements are constantly being reviewed, this week’s meeting of the CIA and MI5 gave added urgency to intensifying protection for London. The simultaneous attacks on three housing compounds used by western workers in Riyadh, and the five suicide teams who struck bars and hotels in Casablanca were seen as evidence that al-Qaeda had reorganised its terror network.

That belief was strengthened last week when its second-in-command, Ayman al-Zawahiri, released a taped message urging al-Qaeda sympathisers to follow the example of the September 11 hijackers. He identified Britain, America, Australia and Norway as targets.

Intelligence chiefs believe that al-Qaeda needs another spectacular attack to show Western leaders that it is back in business despite President Bush’s war against terrorism.

There is already a fear of an attack on Britons living or holidaying abroad and security sources say that a missile attack on a British charter flight was thwarted in Kenya last week. British airline flights to Kenya are still suspended.

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