Britain and the US were on high alert last night amid fears of an imminent terrorist attack.

About 450 soldiers with armoured cars were deployed at Heathrow airport in a move authorised by the Prime Minister and based on intelligence of a specific threat.

Scotland Yard refused to reveal the substance of the danger but terror experts said it pointed to a possible surface-to-air missile attack on low-flying aircraft similar to the one carried out by al Qaida terrorists in Kenya last year.

Commissioner Sir John Stevens said: "This is an alert for the whole of London - what you see at Heathrow is the obvious part of it. We wouldn't do this unless it was necessary."

The fears were heightened last night when Osama bin Laden voiced solidarity with Iraqis, urging them to carry out suicide attacks against Americans.

The message was contained in an audio tape message broadcast by the al Jazeera satellite station.

"We stress the importance of suicide bombings against the enemy, these attacks that have scared Americans and Israelis like never before," said bin Laden.

The TV station said it assumed the tape was authentic because it came through the same means as previous bin Laden statements.

A Foreign Office spokesman said: "We will study the broadcast carefully and consider our response."

In the US, the government warned the public to prepare for a possible chemical, biological or nuclear attack.

Officials advised stockpiling enough food, water and medicine to last at least three days, and to have tape and plastic sheeting to seal windows and doors.

It is the first time the government has offered such stark advice despite several warnings of terror strikes since September 11.

Homeland Security Secretary Tom Ridge said: "We believe the threat has substantially increased in the last couple of weeks."

At Heathrow yesterday, troops with armoured cars moved in at 6am.

Soldiers, dressed in combat fatigues and carrying rifles, patrolled on foot in teams of two or three around the terminal buildings. Armoured vehicles were visible at the main entrance to the airport and parked outside terminals.

Sources said patrols around the airport perimeter would continue for weeks to come.

Later, police began stopping vehicles under the flight path near the village of Wraysbury in Berkshire, a few miles from the airport.

Security was also beefed up at potential targets linked to London's transport and economic infrastructure.

Scotland Yard said: "From time to time it is necessary to raise levels of security activity. We think it is prudent to do so now."

The Ministry of Defence said the troops would remain at Heathrow "for as long as the Metropolitan Police requires our assistance".

Chris Yates, an expert with the specialist journal Jane's Airport Review, said: "The key here is perimeter security - we can put all sorts of technology and security in airports but it doesn't take an Einstein to work out that you can park around the perimeter and launch a missile."