The UK's security alert was sparked by intelligence suggesting a terrorist plan to shoot down a plane.

The BBC has learnt that security services received information at the start of the week about a missile attack at Heathrow.

The following morning, 1,700 police officers and 450 troops were deployed to Heathrow Airport, while armed police patrolled other UK airports.

An Algerian network of hardened fighters, linked to al-Qaeda, is believed to be behind the plan.

We know that al-Qaeda will try and inflict loss of human life and damage upon the UK David Blunkett

The timing of the attack was likely to be this week, to coincide with the end of the Muslim festival of Eid.

The perpetrators are likely to be combat veterans who served in the Algerian civil war and trained in Afghanistan.

This alert was heightened by a statement claiming to be from Osama bin Laden two days ago, which has been interpreted as a call to arms.

'Gang at large'

At 1800GMT on Thursday, BBC security correspondent Frank Gardner said: "As of one hour ago, they had not managed to arrest any of the plotters who are believed to be involved in this plot to bring down an aircraft using a surface-to-air missile.

"And that's making them very worried - these people are still out there."

Home Secretary David Blunkett told the Commons: "We face a real and serious threat.

Heathrow flights continued amid the security

"We know that al-Qaeda will try and inflict loss of human life and damage upon the UK."

But he said giving a "running commentary" on intelligence would hamper anti-terrorist efforts.

This highlights the huge debate in government about how to handle sensitive information.

BBC political editor Andrew Marr called it an "appalling dilemma".

He said ministers could either adopt a heavy-handed approach and be accused of propaganda or sit tight despite information suggesting an attack.

Closing Heathrow was ruled out because it would have been catastrophic and handed victory to the terrorists, Mr Blunkett said.