The government has given the starkest warning yet about the nature of the terrorist threat facing Britain.

For a second day, troops with armoured vehicles have been patrolling Heathrow Airport after warnings that sites in London could be targeted.

Home Secretary David Blunkett said the intelligence information made him so concerned, he felt he had to call in the military.

But closing one of the world's busiest airports had not been seriously considered, he said.

We hope we can get through the next few days without an incident David Blunkett Home Secretary

Metropolitan Police commissioner Sir John Stevens said this was the largest operation of its kind that the force had carried out.

He said the 450 troops and 1,000 extra police officers at Heathrow were the most visible part of increased security in the capital.

Officers assisted by Surrey Police are stopping vehicles under Heathrow's flight path on roads and check points have been set up in villages within eight miles of the airport.

At one point on Wednesday, the Labour Party Chairman, John Reid, said the threat was of the same nature as the attacks on New York on September 11th.

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He angrily denied the deployment of troops may have been political spin, saying: "This is not a game.

"This is about a threat of the nature that massacred thousands of people in New York".

But he later said his comments had been taken out of context and he meant the threat to London was "of the nature" of the New York atrocities.

BBC political editor Andrew Marr underlined what he described as an "appalling dilemma" for ministers who - with a mountain of intelligence to sift through - were caught between over-reacting to threats or not doing enough to prevent an attack.

He said: "We have a political blame culture and an unquantifiable threat. No wonder ministers are jittery."

'Inform the public'

Shadow home secretary Oliver Letwin and Liberal Democrat home affairs spokesman Simon Hughes have written a joint letter to Mr Blunkett, calling for Parliamentary time to be set aside to discuss the current situation.

They said: "We both believe that - whilst the actions being taken are entirely justified - it is important that the public be informed by all parties in the House that this is not a stunt and that it should be taken seriously."

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Earlier, responding to reports Heathrow could be closed, Mr Blunkett said that would have been catastrophic to Britain's trade and industry.

And those "who are threatening us would have been the victors", he added.

Meanwhile extra police have been drafted in at other airports, including Manchester, Stansted, Leeds Bradford International and Jersey.

Authorities have not revealed the precise nature of the threat to London.

It is thought there were intelligence concerns al-Qaeda may try to launch an attack on civilian aircraft using surface-to-air missiles in the UK or US this week.

Extra checkpoints have been set up

The possibility of an attack in Britain is being linked to the end of the Muslim religious festival of Eid, which runs until Saturday.

The tighter security comes as a fresh call was made for terrorist attacks on Western targets, in a recording attributed to Osama Bin Laden.

The BBC's security correspondent Frank Gardner said Western intelligence had "woken up" to the possibility al-Qaeda or people linked to al-Qaeda could try to use surface-to-air missiles to bring down an aircraft.

It follows an attempted attack in Kenya last November and another similar incident in Saudi Arabia last June.