Police have received a top-level order to adopt a "shoot to kill" policy against suicide bombers on the British mainland.
According to reports, London Metropolitan Police Commissioner John Stevens issued the command after intelligence reports from the FBI emerged forecasted that Osama bin Laden's al Qaeda terrorist unit was poised to strike central London.
Hundreds of armed officers are guarding key buildings around the city. Police will receive training on how to identify potential suicide bombers.
There are suggestions that police may adopt an "ask questions later" approach in a bid to stamp down hard on the menace.
In January, police seized the deadly poison ricin from an apartment in north London and foiled a plot to attack Heathrow Airport a month later.
Intelligence passed to Sir John suggested there are more al Qaeda agents in the UK than previously believed.
Security at Heathrow Airport and Westminster has been stepped up.