The public has been warned to be vigilant for terrorism after security was reportedly tightened at transport hubs in the UK.
Extra police were said to have been deployed on the rail network and at Heathrow airport amid "intelligence chatter" over possible attacks.
But Scotland Yard and the Home Office stressed that the overall threat level in the country had not been raised to the highest "critical" level - which would mean an incident was expected imminently.
Unconfirmed reports suggested transport police had been told to cancel days off on Friday.
A Yard spokesman said: "The threat level to the UK is at severe, which means that an attack is highly likely, and has been since January 2010.
"We will police accordingly and use a range of covert and overt tactics which remain under constant review."
The continued speculation about attacks served as a "stark reminder of the real and enduring threat we continue to face from terrorist groups", he added.
A Home Office spokesman added: "There is a continuing need for everyone to remain vigilant and to report any suspicious activity to the police."
BAA, which runs Heathrow and other terminals, said: "Security at our airports remains at a high level and we remain vigilant at all times."