UK aviation industry under 'unclear' threat from al-Qaida

The warning, delivered through a letter, said that no changes to security will be made and the planned method of attack is as yet unclear. Transport officials declined to comment or say when the letter was sent, only issuing a statement saying transport security "is kept under constant review."

The letter fuelled already-heightened concerns at airports and railway stations. Asked about security, British Transport Police declined to comment on the deployment of officers and said the public would not notice significant changes. However, police denied a Sky News report which claimed that officers had been ordered to cancel days off.

Speaking anonymously, a government security official said the heightened awareness came from increased "chatter" among suspects under surveillance by the authorities, but that no threat was imminent.

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 however has not changed since January 2010.

Minor day-to-day revisions of policing are standard practice to handle threats 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 US State Department advised American citizens living or travelling in Europe to be wary amid reports that terrorists were planning a "Mumbai-style" attack on a European city.