The security threat level to the UK as a whole was raised to “severe” a year ago The terrorist threat level specific to major UK transport hubs has been raised from substantial to severe, the BBC understands.
The move includes airports and London railway terminals, although there is no suggestion of any intelligence of an imminent attack.
The threat to the UK overall remains where it has been for the past year at the second-highest level, “severe”.
Security officials are stressing the change is precautionary.
The overall national threat level at severe means a terrorist attack is highly likely.
Beneath this are a series of threat levels for specific sectors of the national infrastructure which are not normally made public.
But it is understood the threat level for major transport hubs, including airline terminals and major railway stations in London, has been increased from substantial to severe.
Officials say if there was any intelligence of an imminent threat or a plot under way the threat level would be raised to its highest level, “critical”.
There have been concerns in recent months over the possibility of Mumbai-style gun attacks in Europe and the intelligence that led to this change is believed to cover Europe as a whole.
In practice, the move means more police are likely to be visible at airports and railway stations from Friday.
BBC security correspondent Gordon Corera said: “Officials are stressing that there is no intelligence of an imminent attack. This is more precautionary than anything else.
Source: Home Office
“If there was some kind of intelligence of a plot under way, or that there was a threat to these locations tomorrow, then the threat level would go up to the highest level which is ‘critical’. That’s not happening.
“These sector threat levels do change quite often. Normally the changes happen out of the public eye and officials don’t comment on them.
“But what we can expect to see is a greater police presence, particularly at airports and large railway stations.”
A Scotland 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 Home Office said there was a “continuing need for everyone to remain vigilant and to report any suspicious activity to the police”.
A spokesman for airports operator BAA said: “Security at our airports remains at a high level and we remain vigilant at all times.”
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