450 troops guard Heathrow

More than 450 troops with armoured cars were patrolling Heathrow today amid fears of an imminent terror attack.

Scotland Yard said the soldiers were part of a "strengthening of security" in London as Britain was put on full-scale alert.

More than a thousand police were also deployed on high-visibility patrols, many of them at the airport and other potential targets in the capital.

It is understood the security services have specific intelligence about a threat to Heathrow. In a statement, Scotland Yard said the "precautionary" action was linked to fears that al Qaeda could use the end of the Islamic "festival of sacrifice", Eid al-Adha, which runs from tomorrow until Saturday, as a trigger for attacks.

Tony Blair's official spokesman said: "The Government and the security authorities are taking whatever action is necessary to protect the public."

The last time the Army was deployed at Heathrow was in 1994 following IRA mortar attacks. The Ministry of Defence said that during the current operation soldiers would be acting in support of Metropolitan Police security arrangements at the airport.

The troops include soldiers from the 1st Battalion, Grenadier Guards and the Household Cavalry Regiment. Sources said they were expected to continue patrols around the 14-mile perimeter for a number of weeks.

The action follows the decision by the US government to raise its level of alert last week amid fears of an imminent terrorist attack.

The Prime Minister also recently warned that an al Qaeda attack on Britain was "inevitable". His spokesman said today it was difficult to comment on the deployment for "good operational and security reasons" but added: "As the Prime Minister has made clear the Government has to take judgments depending on the circumstances.

"It should therefore come as no great surprise that threat levels will rise and fall from time to time."

Meanwhile heavily armed troops also descended on Windsor today. Foot patrols scoured the town and targeted wooded areas of Windsor Great Park.

In November, a statement authorised by Home Secretary David Blunkett warned of "ever more dramatic and devastating" terror attacks, but avoided mention of specific threats.

Security sources have predicted a range of possible threats from chemical attacks to more conventional car bombs. In recent months police and security services have been carrying out an unprecedented operation throughout the UK.

Today it emerged that an al Qaeda suspect was arrested by armed Special Branch detectives as he arrived at Waterloo on a Eurostar train from Paris. The 25-year-old Algerian is suspected of planning to carry out a bomb attack in the UK and is believed to be connected to a gang allegedly involved in the manufacture of the deadly poison ricin in Wood Green.

The man was arrested following a joint operation between French and British security services and is being held at the high-security Paddington Green police station.

At Heathrow, troops began deploying along the perimeter and in and around the terminal buildings at 6am. Soldiers in combat fatigues and carrying rifles patrolled on foot in teams of two or three, rarely staying in one place for more than 20 minutes.

Armoured vehicles and army trucks were at the main entrance, parked outside terminals and spotted stuck in traffic on the network of roads.

Scotland Yard said soldiers were under the direction of police, who were controlling their movements from a special operations room, and the deployment related to "a potential threat to the capital".

"From time to time, it is necessary to raise levels of security activity. We think it is prudent to do so now," a statement said. "The current strengthening of security is precautionary and is related to action being taken in other countries and the possibility the end of the religious festival of Eid may erroneously be used by al Qaeda and associated networks to mount attacks. The use of troops is part of a long-standing contingency, authorised by the Government."

Most passengers were already aware from news bulletins that soldiers had been deployed.

Helen Faulkner, waiting for a flight to Newark at Terminal 4, said she was worried about the risk of an attack. "I am trying not to think about it," she said. "We live with it all the time, so you just have to get on with your life.you just have to keep your eyes open."

On Millennium Eve, US authorities foiled an al Qaeda plot to blow up Los Angeles airport. In recent months BAA has recruited hundreds of extra security staff at Heathrow. One possibility is that suicide bombers could attempt to ram a truck bomb through the perimeter fence of the airport.

At Windsor Great Park, soldiers carrying SA80 rifles were deployed at all entrances and in wooded areas. An area of woodland close to the homes of Sir Elton John and golfer Nick Faldo was searched by troops. One local said: "I've heard on the news that terrorists might be planning to launch Stinger missiles."

Flights from Heathrow pass over the Windsor area. All vans and trucks going into and out of Windsor were stopped at armed police checkpoints.

Meanwhile the US government has warned the public to prepare for a chemical, biological or nuclear attack as the national terror alert remains on "high".

Officials advised stockpiling enough food, water and medicine to last at least three days, and to have duct tape and plastic sheeting to seal windows and doors. It is the first time the government has offered such stark advice and Homeland Security Secretary Tom Ridge said the alert was the "most significant" since the 11 September attacks.

Washington raised the terror threat last week from yellow to orange, indicating a "high" risk of attack. Only red, indicating a "severe" threat, is higher. A classified FBI advisory sent last week to field offices across the US warned that "al Qaida is set to attack".