David Blunkett is to deliver a statement to MPs on the security threat at London's Heathrow airport following pressure from opposition parties.

The home secretary will respond to an urgent question from his Conservative opposite number Oliver Letwin.

It is important the public be informed... that this is not a stunt and that it should be taken seriously Joint opposition letter

Mr Letwin 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.

They wrote: "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."

Nimrod

The move 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

Click here for factfile

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 Letwin told BBC's Newsnight on Wednesday the military response required better explanation in order to avoid "muddled" messages.

"This is not going to be the last time there is a serious threat, but we have to learn as a nation and the government has to learn as a government, how to present things so there is not panic but there is clarity," he said.

'11 September'

Liberal Democrat home affairs spokesman Simon Hughes told BBC Radio 4's Today programme that the best forum for that explanation from the government was Parliament.

Although he added: "It is better that we are prepared and it is better that we take precautions that may in the end not be necessary."

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

On Wednesday Mr Reid 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".

"I was attempting to make clear this is not some sort of game, it's not some sort of PR exercise," he said.

The prime minister's official spokesman said the decision to deploy troops had been taken solely with the aim of protecting the public.

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.

When tough things and remarkable things are done, people will rightly expect a proper explanation Oliver Letwin

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.

He denied "absolutely" that the exercise was propaganda, saying: "We're not in the business of scaring people or doing things without reason."

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