The government was yesterday criticised for failing to present a consistent message about the scale of the threat from international terrorism.

John Reid, the Labour Party chairman, compared the threat at Heathrow to the September 11 attacks in America.

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

However, Reid later backtracked and said that his remarks had been "misinterpreted".

Home secretary David Blunkett also expressed his hope that Britain gets through the next few days "without incident".

It emerged that the government had considered closing Heathrow this week.

But ministers decided that shutting the airport would have inflicted "catastrophic" damage on the economy.

Troops with armoured vehicles spent a second day patrolling Heathrow while some sources had claimed that fighter jets were patrolling the skies above London.

Today's Guardian reports that the terror alert was caused by high-quality intelligence that Islamic extremists with links to al Qaeda have smuggled portable Sam-7 anti-aircraft missiles into Britain.