British Transport Police cancelled leave and called in extra officers after intelligence was received that terrorists could be planning attacks.
A security source said there was no “imminent” threat and the overall threat level had not changed but there was activity from one of a “handful” of extremist cells that cause concern at any time, leading to an adjustment in policing levels.
It is understood that “intelligence chatter” suggested that transport hubs in London could be a target. It was unclear last night whether the terrorists were discussing suicide bomb attacks, or Mumbai-style shootings, or a combination of methods.
A Whitehall source said: "The assessment of the threat to transport hubs has been revised. The view is that there is a greater risk than before, so steps are being taken as a precaution."