Britain's media said the security threat levels for transport hubs has been raised as governments in several European countries warn of a heightened risk of terrorism.
Police were on patrol at major hubs like St. Pancras and King's Cross train stations in what seemed to be an exercise in high visibility policing aimed at reassuring the public.
British Transport Police declined to comment on the deployment of officers, but denied a Sky News report that officers had been ordered to cancel days off.
Britain's government said the overall threat level from international terrorism remains at "severe" - the second-highest level, meaning an attack is highly likely. The level has not changed since January 2010.
Britain's Home Office said that any imminent, serious threat to public safety would prompt a change in the overall threat level - and would see it raised to "critical," the highest point on the system's five point scale.
CBS News security sources in London also said there was no immediate, credible threat suggested by the most recent intelligence available.
Minor day-to-day revisions of policing are standard practice to handle a threat from terrorism that Prime Minister David Cameron said in December "is as serious today as it ever has been."
Britain's move comes after France and Germany increased security after warning of a heightened threat of terrorist attack. In October, the U.S. State Department advised American citizens living or traveling in Europe to be wary amid reports that terrorists were planning a "Mumbai-style" attack on a European city.
The next month, German authorities ordered increased security measures, including a beefed-up police presence in railway stations, airports and other public places. France also especially vigilant on terrorism-linked matters following warnings in the last few months that French citizens were targets for al Qaeda. France's terror alert status is at its second-highest level.
More than 170 people were killed in a 2008 gun attack on multiple targets in the Indian city of Mumbai.
"There is a continuing need for everyone to remain vigilant and to report any suspicious activity to the police," the Home Office said in a statement.
BAA, which runs Heathrow and five other terminals, said security at its airports remained at a high level and that the company is vigilant at all times.
Last month, nine men suspected ofLondon and the London Stock Exchange were charged with terrorism offenses, following the largest anti-terror raid in two years.
Britain had not seen a terror attack since the October warning, but a Dec. 11 attack in Stockholm, Sweden was linked to the U.K.
Taimour Abdulwahab, who blew himself up on a busy street in Stockholm, injuring two people, had lived and studied in Luton, about 34 miles northwest of London, for years before killed himself in the attack.