The threat of a terrorist attack on a major UK transport hub has been raised from “substantial” to “severe”.
Extra police are to be deployed to major rail stations in London, while staffing levels have been increased at London Heathrow airport, according to reports.
However, police have been quick to suggest the move is purely precautionary, with no information suggesting an attack was imminent.
At such a time, the threat level would be raised to “critical” police sources have confirmed.
A Scotland Yard spokesman said: “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”.
The threat level for the United Kingdom as a whole remains at “substantial” – which means a terrorist attack is a “strong possibility”.
A spokesman for airports operator BAA said: “Security at our airports remains at a high level and we remain vigilant at all times.”
Concerns have been raised in recent months over possible Mumbai-style attacks in Europe, including the UK.
Terrorists killed 166 people in a series of gun and grenade assaults in India in 2008.
Germany’s Reichstag cupola was closed to tourists last year, following information of a possible attack on the city.
In October last year, Britain and the US upgraded their travel advice for Europe in response to fears of similar attacks and a number of bomb alerts on the continent.