The UK’s terror threat level has been raised from “substantial” to “severe”, meaning an attack is now judged to be “highly likely”.
Home Secretary Priti Patel described the move as a “precautionary measure” and said it was “not based on any specific threat”.
“The public should continue to remain vigilant and report any suspicious activity to the police,” she added.
On Monday night, four people were killed in an attack in Austrian capital Vienna.
Last week, three people died after a knife attack in Nice, France, while teacher Samuel Paty was murdered in Paris last month.
JTAC is based at MI5’s headquarters in London and is made up of counter-terror experts from the police, government and security agencies.