The terror threat at Heathrow is serious but the public should be "alert, not alarmed", said David Blunkett.

The home secretary was speaking in the House of Commons after bowing to pressure from opposition parties to deliver a statement on the security operation at the world's busiest airport.

The terrorist must not be able to assess what we know or how we know it David Blunkett

He said he made it clear last year that there there would be heightened security at airports and other key transport hubs and that there were terrorists intent on taking British lives and damaging the UK.

He said that sometimes security would have to be further increased to match a particular threat.

"That is precisely what we have done this week," he said.

No undermining

However he added: "I do not believe it would be responsible to provide a running commentary [of the ongoing security situation] from the Despatch Box.

"We must do nothing to undermine the work of the the police and security services.

"The terrorist must not be able to assess what we know or how we know it."

Just a few minutes earlier Tony Blair's spokesman had said: "This is a serious issue being dealt with seriously by the government."

Oppostion call

Asked whether people should think of cancelling their holidays the spokesman said: "If the Government thought it was appropriate to issue advice or close infrastructure it would do so. It hasn't."

Mr Blunkett echoed that sentiment saying that his son had flown out of Manchester Airport on Thursday morning.

Terror alert comes to flightpath villages - click here for eyewitness story

"It is safe to use our airports... people can go about their business free from fear," he said.

Mr Letwin said he backed the security measures but was concerned at what he said were "confused and conflicting signals".

The Tory spokesman had joined forces with Lib Dem counterpart Simon Hughes on Wednesday when they wrote a letter asking for time to discuss the current situation in the House of Commons ahead of the half-term recess.

Mr Hughes later issued a statement following Mr Blunkett's appearance in the Commons.

"Nobody would expect ministers to reveal their information, sources and tactics," he said.

"But the British population must be kept informed and it should be made clear what further action they may need to take."

Nimrod

Pressure for a statement followed a day of confusion and scepticism, during which ministers angrily denied the exercise was a publicity stunt ahead of a war against Iraq.

There are 450 troops and about 1,700 extra police officers patrolling at Heathrow - guarding key sites and stopping vehicles under flight paths within about eight miles of the airport.

A Nimrod is keeping watch over London

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The Ministry of Defence has confirmed a single Nimrod maritime surveillance aircraft has been flying over London, "to aid communications on the ground".

Labour chairman John Reid at one point on Wednesday appeared to compare the threat to the 11 September attacks, before saying his remarks had been "misinterpreted".

Mr Blunkett backed that claim suggesting that his colleague had been misquoted by the media.

'11 September'

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

But he later clarified his remarks, saying he had been "misinterpreted".

The government and police said the ongoing security alert was linked to fears that al-Qaeda could use the end of the Muslim festival of Eid, which runs until Saturday, as a trigger for an attack on London.

Police have not ruled out troops being deployed in the centre of London at a later date.

Mr Blunkett said he had considered, but decided against closing the world's busiest international airport.

Metropolitan Police Commissioner Sir John Stevens said it was the largest operation of its kind that police had ever been involved in and he denied it was a propoganda exercise.

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