British officials consider a terrorist attack "likely" when the threat level is substantial, and "highly likely" when it is severe.

The decision was made by the Joint Terrorism Analysis Centre (JTAC), which conducts a formal review of the threat level every six months, and taken independently of government ministers.

The level was raised from substantial to severe in August 2014 in response to conflicts in Syria and Iraq.

In 2017, the threat was briefly raised on two occasions to "critical", the maximum level indicating an attack is "highly likely in the near future", before returning to severe.

Copyright Agence France-Presse, 2019