Chief Superintendent John Bunn, second in command at the anti-terrorist branch, said 800 leads were being followed up and about 24 names being checked out.

A team of four detectives is still in the United States liaising with the FBI and following up the information it has provided.

Lotfi Raissi is awaiting extradition to the US and is alleged to have trained some of the hijackers.

Habib Zacarias Moussaoui, who lived in Brixton, south London, is alleged to have been the "20th hijacker" and is in custody in the US.

Mr Bunn said the third man had been arrested in the West Midlands and subsequently released, but was still under investigation.

He added that many of the suspects might turn out to be entirely unconnected to the terror attacks on 11 September.

Anthrax hoaxes

He said a number of anthrax-related hoaxes were also being investigated. On Wednesday Home Secretary David Blunkett said hoaxers could face up to seven years in jail.

Since the attacks there have been more than 2,000 calls to the police's anti-terrorist hotline.

Scotland Yard is improving its secret, alternative command control centre in case its headquarters in central London are attacked by terrorists.

"The main thing that came out of it for us - there were lots of lessons - was that there is a need for a back-up command control centre," he said.

"The emergency control centre in New York was in one of those towers and when it went down they had great difficulty communicating for four to five hours.

"All their communications, even the telephone networks, were out. We have always had a standby place that we wouldn't want to disclose.

"We have got generators and we can set up a command and control centre - what we have to look at is what that command and control centre is in the light of the enormity of 11 September."

Strain

He added: "There is a need for us to look at our evacuation planning and we are doing that in relation to some of the tall buildings in London."

In London the number of extra police officers on the streets has now fallen to 500 from a peak of 1,500. It could rise again on Muslim prayer days when extra officers are patrolling near mosques.

Sir John said officers had been working "flat out" and there was concern that some might feel the strain.

"We don't want armed officers to be working 12-hour shifts," he said.

Some officers have been doing 16-hour shifts and sleeping in vans and police stations.

"For me that's not satisfactory," he said.