A security official said a threat wasn't "imminent" but information on activity from one of a "handful" of extremist cells caused concern, leading to an adjustment in policing levels, The Daily Telegraph reported.
Officials said "intelligence chatter" indicated transport hubs in London could be targets. The Daily Telegraph said it was unclear whether the terrorists were discussing suicide bombings, shootings or a combination of methods.
"The assessment of the threat to transport hubs has been revised," a security official said. "The view is that there is a greater risk than before, so steps are being taken as a precaution."