London: Britain has raised its terrorist threat level for major transport hubs, including airports and London railway terminals, from substantial to severe, a report said on Friday.
There was no suggestion of any intelligence of an imminent attack and security officials are stressing the change is precautionary, said the BBC.
The overall terror threat to Britain remained at the second-highest level of "severe" on Friday, where it has been for around a year and which means an attack is considered highly likely.
The highest level in the five-stage grading is "critical" which means an attack is expected imminently.
Beneath this overall system are threat levels for particular sectors of the national infrastructure which are not normally made public, reported the BBC.
But the broadcaster said the threat level for major transport hubs, including airline terminals and major railway stations in London, had been increased from substantial to severe.
The BBC said the threat levels for specific sectors change quite often and are normally kept out of the public eye.
The latest change meant there was likely to be a greater police presence at airports and railway stations on Friday, added the broadcaster.
Sky News television and The Daily Telegraph newspaper earlier reported that transport police had cancelled leave for their officers and called for back-up to help deal with the heightened alert.
London`s Metropolitan Police 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."
A spokesman for the Home Office, or interior ministry, added: "There is a continuing need for everyone to remain vigilant and to report any suspicious activity to the police."
Airport operator BAA, which runs London Heathrow and other terminals, said: "Security at our airports remains at a high level and we remain vigilant at all times."
The reports of heightened security came after a December 11 suicide bombing in Stockholm was linked to Britain, with the bomber having lived in Luton, just north of London, for the past few years.
It also came after nine men were charged at the end of last month with planning attacks, reportedly in a pre-Christmas plot with a string of high-profile targets including the London Stock Exchange and the US embassy.