The UK's terror threat level has been raised from "substantial" to "severe", meaning an attack is considered to be "highly likely".
The Joint Terrorism Analysis Centre (JTAC), which is based MI5's headquarters in London and consisted of counter-terror experts from the police, government, and security agencies, made the decision to increase the threat level on Tuesday.
Home Secretary Priti Patel described the move as "precautionary" and not in response to any specific threat.
The measure follows an attack on Monday night in Vienna that saw the deaths of at least five people at the hands of a gunman.
After the attack in the Austrian capital, Prime Minister Boris Johnson said he was "deeply shocked".
"The UK's thoughts are with the people of Austria - we stand united with you against terror", he said in a tweet.
Last week, three people were killed in a knife attack in Nice, France, and teacher Samuel Paty was murdered outside Paris last month after showing a cartoon of the Prophet Muhammad in his classroom.
The terror threat level in the UK has been "substantial" since November 2019, after it was lowered from "severe" for the first time in five years.
A "severe" threat level is the second-highest, behind "critical", which was last reached in May 2017 following the Manchester Arena bombing.