Yard may shut Heathrow

Scotland Yard chief Sir John Stevens said today that he is prepared to close Heathrow Airport if there is an "imminent" terrorist threat.

The airport is already at the centre of a massive security operation involving troops, amid fears that terrorists could try to shoot a jumbo jet out of the sky using a surface-to-air missile.

At a special briefing at New Scotland Yard, Sir John refused to give details about the present threat to Heathrow. He did, however, admit that the possibility of closing the airport was an option, while senior officers said police were being deployed along flight paths into Heathrow.

Sir John spoke as it was confirmed that ministers had seriously considered a total shutdown of Heathrow under the current state of alert. They decided not to take the ultimate step only because of the "catastrophic" impact on Britain's links with the world - and because it would have been a surrender to terrorism.

Other key details to emerge today are that:

* Police have mounted the biggest security operation ever seen, involving up to 2,000 of their officers and soldiers in a "pan-London operation" to protect targets in the capital;

* Secret government papers show "it is a certainty" that there are still groups and individuals at large in Britain who pose a " present and real" terrorist threat to public safety at home and abroad.

Sir John would not give details of other possible targets in London, but said that the Met is working closely with British Transport Police to protect the Tube.

As armed police and the Army maintained a ring of steel around Heathrow today, Home Secretary David Blunkett made clear that the closure of the world's busiest international airport had been seriously contemplated.

In clear confirmation of the fears of a missile attack on an incoming or outgoing flight, Mr Blunkett said that measures were now in place to watch passengers and protect aircraft, "particularly in terms of aircraft landing". The Home Secretary continued: "We hope we can get through the next few days without an incident. I hope we can."

Sir John Stevens said the present level of threat at Heathrow, though higher than the one facing the rest of London, was still not described as "imminent" - the highest level when security forces had identified suspects planning an immediate attack.

Troops could be deployed to guard central London in the event of an emergency terror threat, he said, but there were no plans to do so at present.

Details of the thinking behind the high-profile defences came as it emerged today that a genuinely independent assessment of secret Government papers shows a real and present threat.

Lord Carlile QC, the leading barrister appointed by the Government to examine the fairness of the new emergency terrorism laws brought in after 11 September, made absolutely clear that there are both foreigners and UK citizens plotting terrorism.

"From the material I have seen, it is a certainty that there remain in the United Kingdom individuals and groups who pose a present and real threat to the safety of the public here and abroad," he said.

Meanwhile, Labour Party chairman John Reid got into a muddle today after comparing the danger facing London with 11 September.

Asked if the deployment of troops at Heathrow was an over-the-top reaction as a war against Iraq looms, he had angrily replied: "This is not a game. This is about a threat of the nature that massacred thousands of people in New York."

But he later downplayed his warning and claimed his original remarks had been "misinterpreted".

Dr Reid told Radio 4's World At One: "It was precisely the suggestion that any government, this Government included, would use such a serious subject as international terrorism which gave us such things as New York for their own purposes of spin or public relations, I just found pretty contemptible.

"Now that was misinterpreted, I can understand why, by someone thinking that I had said that the scale of the threat at Heathrow was the same as the scale of the threat in New York. That was not meant. I am glad to have the chance to clear that up."