A number of people are feared dead and many injured following explosions at the airport and a metro station in the Belgium capital.
It is not yet known which specific areas are regarded as “key locations” for increased policing presence, which is being used to protect the public and provide reassurance.
Mark Rowley, leading Counter Terrorism for National Police Chiefs’ Council, said: “Our thoughts are with the people of Brussels following this morning’s horrific attacks.
“As a precaution forces across the UK have increased policing presence at key locations, including transport hubs, to protect the public and provide reassurance. This is not in relation to any specific information or intelligence.
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“The threat to the UK from international terrorism remains at severe as it has been since August 2014, meaning an attack is highly likely.”
He added: “In London specifically, the Metropolitan Police Service has mobilised additional officers, who will carry out highly visible patrols at key locations around the Capital including the transport network.The number of officers deployed will be regularly assessed.These additional officers are deployed as part of reassurance measures.
“The police presence across London and the rest of the UK is constantly under review
“We are in close liaison with the Belgium authorities and will continue to monitor the situation.”
The public is being urged to be “alert but not alarmed” and report anything suspicious to the confidential Anti-Terrorist Hotline on 0800 789 321. In an emergency always call 999.”