Britain should be prepared for an imminent terror attack, the government has warned.

The Labour Party chairman said Britain is 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."

But there appears to be controversy about the exact scale of the threat. Dr Reid later moved to clarify his remarks, saying he did not intend to say that a repeat of September 11 was being planned.

However the government insisted that the public must be vigilant at a time of high terror alert.

David Blunkett also revealed that ministers had considered shutting Heathrow airport on the basis of intelligence information.

As the huge police and armed forces operation at the London airport entered its second day, the home secretary said closing the airport would have been a victory for terrorists and "catastrophic" for Britain's economy.

He said the military presence at airports should reassure the public rather than alarm them.

"People can feel safer in terms of the measures we have taken than under normal circumstances where no threat had been perceived," he said.

Speaking during prime minister's questions, Tony Blair sent his "thanks and congratulations to the police, our armed forces and the security services who protect us against the security threats that do not just affect this country but affect countries right across the world at this moment".

"We occasionally do have to take measures that we would prefer not to take in order to give people the protection and security they need," the prime minister said.

Heightened security measures have been extended from Heathrow to other airports amid fears of an al Qaeda attack on a passenger jet.

While soldiers and armed police continue to guard the London airport and its flight paths, it emerged on Tuesday that armed police have begun carrying out spot checks on cars at Manchester airport.

The moves came amid concern that al Qaeda could mount an attack to coincide with the end of the religious festival of Eid.

Manchester police said the latest move was aimed at reassuring the public rather than dealing with a specific intelligence threat.

"This is part of our ongoing operations at the airport which aim to reassure the public that every possible step is being taken to make sure people are safe while travelling," said chief inspector Martin Gaffney.