Undercover officers posing as builders and a high-tech 3D database of Olympic venues are among plans to protect the 2012 Games against terrorists.

The London Olympics will be a "huge target" for terrorism, the capital's police chief Sir Ian Blair said yesterday.

All 200,000 people working at the Olympics will have their passports and fingerprints checked in advance. They will include 70,000 volunteers, 40,000 builders, 30,000 security staff and 60,000 others.

Covert officers pretending to be construction and security workers will also spy on their workmates.

A full-time team of officers is already compiling a 3D database with aerial images and maps of all Olympic venues. The computer programme allows them to spin each location 360 degrees and to simulate emergencies.

London's Olympic bid budgeted £220 million for security, but that was before the bombings of July 7 last year. So far, the Metropolitan Police has received £4.6 million for the Games.

The force will appoint a special assistant commissioner to lead the massive operation, Sir Ian said.

"There can be no doubt that the 2012 Games - if the current threat scenario stays the same - will be a huge target," Sir Ian said.

However, there was no specific intelligence about a threat yet, he added.

"I don't want to consider what al Qaeda thinks is the big one. But there is no question that hosting an Olympic Games in a liberal democracy rather than in China poses different issues."

The current head of Olympic security, Assistant Commissioner Tarique Ghaffur, said police would follow a "covert strategy" to prevent risk at the Games. "We need significant knowledge of who will be working there."

Sir Ian's warning came after The Guardian newspaper quoted senior intelligence sources saying Britain is now the main target of al Qaeda.

The July 7 bombings was "just the beginning", one source claimed.

Britain has become an easier target than America, as terror suspects can take cover in its big Pakistani community, the paper said.

Al Qaeda is more dangerous than before, experts added. A loose set of splinter groups has now evolved into organised sleeper cells, each with a leader, explosives specialist, weapons quartermaster and volunteers.