Sir John Stevens was speaking to the MPA

He told a meeting of the Metropolitan Police Authority (MPA) that he could not go into details but that the city was on a "high state of alert at the moment".

"There has been a massive amount of work taking place in the anti-terrorist branch and security services for the last year and a half," he said.

"The activities undertaken by the anti-terrorist branch specifically in relation to London and nationally with the security services has prevented in some instances a possible terrorist attack on this city.

"That continues and we regard it as our major responsibility at this time.

"As the head of the anti-terrorist branch, David Veness says he is two-and-a-half to three times busier now than he has ever been in his extensive experience."

Last month, a mock chemical attack took place at Bank station in the City to assess the emergency services' ability to cope with a terrorist attack.

In April it was announced that more than �300m will be spent on equipment capable of detecting radioactive material being smuggled through Britain's ports.