Fears of a possible terror attack have prompted security officials to deploy hundreds of troops at sites around London.

The decision to boost security in the capital was announced on Tuesday morning, amid concern that al Qaeda or other terrorists could mount an attack to coincide with the end of the religious festival of Eid.

"From time to time, it is necessary to raise levels of security activity. We think it is prudent to do so now," said a statement from the Metropolitan Police.

Over 400 soldiers were being deployed to beef up security at Heathrow airport and other unspecified sites around London.

Number 10 said there were "good operational and security reasons" why the unprecedented move had been made and it was part of the continuing counter-terrorism campaign.

"The government has to take a judgement depending on the circumstances," said the official spokesman.

"It should not come as any surprise that the threat level will rise and fall from time to time.

"The government and security services are taking whatever action they believe is necessary to protect the public."

The move was authorised by the government and the police as "part of a long-standing contingency" for coping with terrorist threats.

"To avoid prejudicing ongoing operations, we do not intend to give any further details of security arrangements, other than to say that this action is in line with the policy of taking whatever action we believe necessary to protect the public," the police statement said.

Defence officials said the troops had been brought in from training exercises around the UK and would be deployed for as long as needed.

The heightened state of alert comes amid fears that the tense situation in the Gulf and Britain's strong support for the US-led "war on terrorism" has made Britain a key target for terrorists.

A series of police raids have indicated that terrorist cells operate in the UK, while the prime minister has warned that a terror attack appears "inevitable".

BAA, which operates Heathrow airport, said security remained its key concern.

"Safety and security for passengers and staff is our top priority. BAA works extremely closely with the Met Police on security issues and we are co-operating fully with these latest enhancements to security," said Mick Temple, managing director at the airport.

"We understand that this is a precautionary measure and we would ask passengers to continue to be vigilant and patient in the light of any delays that may be caused by this or any other additional security measures that may be introduced from time to time."