AL-QAEDA terrorists using the cover of huge anti-war protests are a very real threat to the safety of George Bush during his state visit to London next week, Britain’s most senior policemen have said.

Scotland Yard says that concerns over an attack on the President are the reason for the biggest security operation Britain has seen for any visiting head of state. The threat of demonstrators embarrassing Mr Bush during his three-day stay is a secondary worry.

Senior officers finalising security arrangements with the White House described the trip as “a nightmare”. One source told The Times that the original demands made by the US Secret Service were “simply unacceptable. They wanted to turn London into little Washington by closing roads for miles around, hours before the President’s motorcade passes, and that is just not acceptable here.”

While the White House has been told that it will not get its own way on every demand, police have agreed to greater restrictions on protesters and the public than they wanted.Few people will see Mr Bush, who no longer risks walkabouts or unscheduled stops.

His security team fear that if street protests lead to running battles with police and gridlock in the centre of London, Islamic terrorists could take advantage of the disorder to stage an attack.

Because of the September 11 attacks, no president has been given tighter security and Mr Bush’s bodyguards fear that too much of Mr Bush’s itinerary has leaked out.

Wherever he goes, Mr Bush is surrounded by a tight security cordon, known to the White House as “the bubble”, which forms an impenetrable wall around him. The White House regards London as a high-risk target for a suicide attack. Militant cells of mainly North African extremists are known to operate in Britain.

White House concerns have led to a row over how many of the President’s Secret Service detail, the “Men in Black”, will be allowed to carry guns and on the rules of engagement about when US agents can open fire. They now carry firearms openly in front of American crowds.

Whitehall officials insist that US agents must follow the law and can use arms only if there is “an imminent or direct threat to life”.

The US Secret Service wants immunity from prosecution for any agent who uses a gun and insists that Mr Bush’s personal safety is its responsibility, not Scotland Yard’s.

The White House wanted to bring 250 armed officers with Mr Bush. Diplomats argued for fewer, saying there will be well over 4,000 police on the streets, including many who are armed.

Scotland Yard said last night that it would not seal off the centre of London during the visit, from November 18 to 21. “Some roads will be closed for a time for operational reasons but the Mall and Whitehall will remain open,” a police source said.

Protest organisers say the Metropolitan Police has told them that the 100,000-strong march planned for next Thursday cannot pass down the Mall to Buckingham Palace, where Mr Bush and his wife, Laura, will be staying with the Queen.

Anti-war protesters said that they would resist moves to enforce this “exclusion zone”. Lindsey German, convener of the Stop the War Coalition, said: “It is an outrage that the most unwelcome guest this country has ever received will be given the freedom of the streets, while a movement that represents majority opinion is denied the right to protest in the area which is the heart of government.”

To add to security worries, anarchist groups from across Europe are being urged to gather in London for Mr Bush’s visit after trying to disrupt a European Social Forum summit in Paris three days earlier. Undercover police are trying to infiltrate these groups.

A senior Scotland Yard source told The Times: “We are not so concerned about some anti-war protester throwing rotten fruit at the President. Our worry now is the more dangerous elements who may be here.”

The Mayor of London, Ken Livingstone, yesterday dismissed the idea that wide swaths of Central London will be closed. “The ideas of some American security advisers that perhaps we should shut the whole of Central London for three days, ignoring the economic consequences of that, I don’t think that's got a chance at all.”

The final decision on which roads to close rests with Sir John Stevens, the Metropolitan Police Commissioner, who is in close touch with ministers and can overrule the mayor.

Scotland Yard will reveal road closures only on each morning of the President’s stay. Anti-war protesters want to stalk the President’s every move while he is here but the planned security operation should make this impossible.

The main demonstrations are planned for Thursday when Mr Bush meets Tony Blair in Downing Street and the President lays a wreath at the Tomb of the Unknown Warrior in Westminster Abbey.

The US Secret Service is so worried about the amount of publicity that has leaked out about the President’s intended visit to Sedgefield, Mr Blair’s constituency, that it wanted it scrapped. Foreign Office diplomats hope that the itinerary will be finalised today.

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