MINISTERS in Tony Blair’s government were taken aback by the scale of a military operation to secure Heathrow Airport from an al-Qaida attack, newly released official files suggest.

The then prime minister rubber-stamped the decision to deploy about 450 heavily armed troops, some in armoured vehicles, to create a “ring of steel” around Britain’s busiest airport in February 2003 after intelligence identified it as a likely target for an imminent attack by militants armed with anti-aircraft missiles.

Files released by the National Archives at Kew, west London, show that then home secretary David Blunkett told the Cabinet that while the deployment had been agreed with the security services and the military, they had not been expecting such a high-profile show of force.

“The armed forces had not been expected to use their military hardware quite so visibly and this had, perhaps, led to the threat seeming more dramatic than was in reality the case,” he said, according to the minutes of the meeting.

He said the action had been authorised in response to a “specific terrorist threat” identified by the security services, but did not go into detail.

“The security services were continuing to work on assessing the threat but were having to strike a difficult balance between intervention and surveillance,” he added.

In discussion, ministers dismissed claims the deployment operation was “merely a tactic” to build support for the impending invasion of Iraq by British and US forces as “absurd but not surprising.”

“Unfortunately, greater threats were likely from time to time for the foreseeable future and the government and the public would have to learn not to overreact to them,” the minutes noted.