The level of threat of a terrorist attack in the UK has been decided to be upgraded from "significant" to "serious", ie to the fourth of the five alert levels.
The decision by the Joint Center for Terrorism Analysis was taken after the terrorist attacks in Vienna and France.
The "serious" level of threat means that the authorities consider a terrorist attack on British soil "extremely likely".
The security alert was in the third tier since last November, which meant that a terrorist attack was considered "possible".
The fifth level of terrorist threat is the "critical" and implies that an attack is extremely likely in the near future.
Reacting to the deadly attack in Vienna, British Prime Minister Boris Johnson said he was "deeply shocked", adding that the British were united with the Austrians in the face of terrorism.