Heathrow on alert for attacks
The operation came after around 450 soldiers with armoured cars were deployed at the airport in a move authorised by British Prime Minister Tony Blair and based on intelligence of a specific threat.
Police said Osama bin Laden's network might time attacks to coincide with the end of this week's Muslim Eid al-Adha holiday.
Downing Street said the military deployment was part of "an ongoing operation".
Scotland Yard refused to reveal the substance of the threat but terror experts said it pointed to a possible missile attack on a plane similar to the one carried out by al-Qaida terrorists in Kenya last year.
Last November's plane rocket attack in Kenya saw some 270 passengers and crew escape with their lives when two missiles narrowly missed an Israeli jet taking off from Mombasa airport.
A devastating suicide bombing was carried out at the same time on a hotel used by Israeli tourists in Mombassa.
Fifteen people were killed. Al-Qaida claimed responsibility for both attacks.
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 something like a missile."
Hours after the soldiers arrived, police began stopping vehicles under the flightpath near the village of Wraysbury in Berkshire, just a few miles from the airport. Mr Blair's official spokesman would not disclose details of the "specific threat".
"In terms of these reports, I have seen them. I have got no comment to make. There are good operational and security reasons why we are not commenting in detail on this," he said.
Under questioning, he added: "This is an ongoing operation in relation to a specific threat."
Asked if Mr Blair had discussed yesterday's deployment with Metropolitan Police Commissioner Sir John Stevens, the spokesman replied: "You can take it that the prime minister has obviously been party to this decision."
Meanwhile yesterday, a radical Islamic activist warned people to stay away from government buildings and financial institutions, saying radical Muslims were prepared to act as suicide bombers.
Sheikh Omar Bakri Mohammed, a leader of the small Islamic group al-Muhajiroun, said yesterday that al-Qaida supporters in Britain might carry out attacks.
"I would warn people to take precautions. Do not go into government buildings do not be in any financial institutions, keep away from these locations," he told BBC radio.
Police have arrested several terrorist suspects since the January 5 discovery of the poison ricin in a north London.
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