Home Secretary Priti Patel has escalated the UK terror threat level from “substantial” to “severe”.
The move follows an attack in Vienna that left at least three dead.
The change means that an attack on UK soil is now deemed “highly likely”.
Gunmen – believed by Austrian police to have an Islamist motive – killed three and left several wounded on Monday night as they went on a shooting spree armed with an assault rifle, carrying other handguns, and wearing a fake explosive belt.
It came on the eve of Austria entering a month-long coronavirus lockdown – a move that England will follow on Thursday.
Ms Patel tweeted: “The Joint Terrorism Analysis Centre has changed the UK terror threat level from substantial to severe.
“This is a precautionary measure and is not based on any specific threat.
“The public should continue to remain vigilant and report any suspicious activity to the police.”
Earlier on Tuesday, the Prime Minister has expressed his deep shock at the “terrible attacks” in Vienna.
Mr Johnson said: “I am deeply shocked by the terrible attacks in Vienna tonight.
“The UK’s thoughts are with the people of Austria – we stand united with you against terror.”
