The Home Secretary’s remarks came as 1,450 soldiers and police maintained their guard around Heathrow for a second day, in the biggest security operation at a British airport. The patrols will last at least until the weekend. Security at other airports around the country was also intensified.

Sir John Stevens, Commissioner of the Metropolitan Police, said he could not rule out the possibility that soldiers would also patrol Central London if the terrorism threat increased. “This is a pan- London operation,” he said. “We are asking people not to panic or get alarmed and to be alert.” But he told Londoners to expect more high-visibility operations until the threat had diminished.

Sir John said that the operation at Heathrow had been ordered after information from MI5 and MI6, as well as the police’s Anti-Terrorism Branch. The alert involves other targets in London, which the Commissioner would not identify. Officers armed with Heckler & Koch sub-machineguns patrolled the streets around Whitehall and St James’s Park, close to Buckingham Palace and government offices. Security in Whitehall has focused on small commercial vehicles that could hold bombs.

Public alarm over the scale of the threat facing London was fuelled when John Reid, the Labour Party Chairman, appeared to suggest that it was comparable to the September 11 attacks. “This is not a game,” Dr Reid said. “This is about a threat of the nature that massacred thousands of people in New York.”

He clarified his remarks later, saying that he had been prompted by irritation at suggestions that the alert was a kind of PR stunt and that he had not wanted to compare the threat to London with the attacks on America.

The Prime Minister’s official spokesman also sought to put Dr Reid’s remarks in context, saying: “John Reid was dealing with the very offensive suggestion that this is in some way got up by the Government and by the security services. It is worth pointing out that this is real life, this isn’t a movie, they aren’t extras on a film set. They (the troops) are there to protect the public and the decision to deploy them was taken with that end, and that end solely, in mind. You can think and say what you like about the Government, but do not doubt motives when it comes to something like this. The threat from international terrorism is real and is present.”

Mr Blunkett, explaining why ministers had not gone as far as closing Heathrow, said: “For those who are threatening us it would have been a victory. Trade would have suffered and the transport of people would have been disrupted; this would have been a catastrophic thing to have done.”

Asked about the intelligence that led to troops and tanks being sent to Heathrow, the Home Secretary said: “The immediate threats are not verifiable and therefore we are working on finding out more information. I hope we can get over the next few days without incident. We are confident we can.”

At Manchester and Birmingham international airports, police began spot-checks of passengers and opened checkpoints on perimeter roads, although officials denied that there was a specific threat and said that the measures were to deter an attack and to reassure the public. Security was also tightened yesterday at Stansted, Gatwick, Liverpool’s John Lennon airport and Leeds Bradford airport.

In London, a total of 1,700 officers have been mobilised for security duties, including the protection of mosques in case of a reaction to an attack. But the strain is beginning to show: ten operations against muggers in various parts of the capital have had to be abandoned this week, and reports of street robberies have risen by 5 per cent in recent days.

As security reached its highest level since the September 11 attacks, Jack Straw, the Foreign Secretary, told Labour MPs that he was furious at the way information had been plagiarised for the Government’s dossier on Iraq. His admission was taken as a sign that the Cabinet felt that the affair had weakened the case for war rather than strengthening it. However, the Prime Minister defended what Conservatives have nicknamed the “dodgy dossier” at Prime Minister’s Questions yesterday, saying that it was accurate even though parts were copied from a Californian student’s thesis.

He said: “The part of the document that dealt with Intelligence was indeed from intelligence sources as the document stated. It is important that is underlined.”

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