David Blunkett has defended the government's decision to heighten security at Heathrow airport.

The home secretary said troops had been deployed to provide an "enhanced state of security".

"We face a real and serious threat. We know that al Qaeda will try and inflict a loss of human life and damage upon this country," he said.

He said the decision had been taken to provide "preventative and protective" measures at high risk locations.

"Where threats are specific we seek to thwart them," the home secretary told MPs.

He insisted that the public should not be afraid of using Britain's airports.

"I certainly reassure everyone that it is safe to use our airports and they should not be fearful doing so, precisely because the measures have been put in place and the security provided to ensure they can go about their business free from fear," he told MPs.

But he warned that he would not provide a "running public commentary from the despatch box on every turn and development".

Blunkett - who agreed to appear before MPs after the speaker granted the opposition's request for an urgent question - had earlier accused his opponents of "party political grandstanding".

Shadow home secretary Oliver Letwin said that the government had alarmed the public by sending out a mixed message about the scale of the threat.

He said ministers had set out a "confused and conflicting signal" and called for a single minister to be put in charge of the UK's terror strategy.

The move came after Tony Blair defended the government's response to the terror threat. He said ministers were taking "every precaution we can in order to keep people safe".

Blair insisted that "we cannot and should not start disclosing details of everything we know or may know".

"The only way we can be fully safe is not just taking security measures necessary to protect ourselves...but to make sure, along with other countries in the world, we do absolutely everything we can to root these terrorists out," he told the press in Downing Street.

The government makes its judgments "based on the advice of our security services what the appropriate action is to take", Blair said.

Downing Street dismissed reports that the decision to station tanks at Heathrow was an attempt to win support for the war effort.

"People can say what they like and think what they like about the prime minister but I don't think they question our motives on something like this," said the official spokesman.

"The public understand the changed nature of all our lives after September 11."

Speaking on Wednesday, the Labour Party chairman said the UK was facing the same threat as that which "massacred thousands of people in New York".

Dr John Reid rejected suggestions that the deployment of troops at Heathrow was a disproportionate response to a feared terrorist attack.

"This is not a game. This is about a threat of the nature that massacred thousands of people in New York," he said.

"I am not even going to take seriously those people who suggest this is part of some sort of game."

Dr Reid later moved to clarify his remarks, saying he did not intend to say that a repeat of September 11 was being planned.

"I made no reference at all to the scale of threat," he said in a bid to play down his earlier comments.