The UK's terror threat level has been raised from "substantial" to "severe", meaning an attack is now judged to be "highly likely".
The move follows Monday night's attack in Austrian capital Vienna, in which four people were killed.
Last week, three people died after a knife attack in Nice, France, while teacher Samuel Paty was murdered in Paris last month.
Home Secretary Priti Patel described the action as a "precautionary measure following the terrible incidents we've seen in France last week and the events we saw in Austria last night".
"The British public should be alert but not alarmed," Ms Patel said, as she urged the public to report any suspicious activity to the police.
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The decision to raise the UK's threat level has been taken by the Joint Terrorism Analysis Centre (JTAC).
JTAC is based at MI5's headquarters in London and is made up of counter-terror experts from the police, government and security agencies.
UK terror threat level raised to 'severe' - meaning attack is 'highly likely'
5 years ago