Armed police officers stand guard outside the Houses of Parliament in London, Britain, on Nov. 4, 2020. Britain on Tuesday raised its terrorism threat level from "substantial" to "severe", meaning an attack is highly likely. (Photo by Ray Tang/Xinhua)

LONDON, Nov. 3 (Xinhua) -- Britain on Tuesday raised its terrorism threat level from "substantial" to "severe", meaning an attack is highly likely.

The Sky News reported the decision to raise the threat level has been taken by the Joint Terrorism Analysis Centre (JTAC), part of MI5, the British Security Service.

British Home Secretary Priti Patel described the move as a "precautionary measure", saying that it was "not based on any specific threat".

"The public should continue to remain vigilant and report any suspicious activity to the police," she said.

The move came after four people were killed in a shooting by a suspected terrorist in Austrian capital Vienna on Monday night.

Last week, three people died after a knife attack in Nice, France, after a teacher was murdered in Paris last month.

The terror threat level in UK was at "substantial" since last November.

The "severe" level is the second-highest level, with only "critical" above it, which was reached in May 2017 after the Manchester Arena bombing, in which 22 people, including a number of children, were killed and hundreds injured.

The attack happened when Salman Abedi detonated a bomb at the end of a concert by U.S. pop singer Ariana Grande at Manchester Arena. It was the worst terror attack in Britain since the London bombings in 2005 in which 52 people died.

Armed police officers stand guard outside the Houses of Parliament in London, Britain, on Nov. 4, 2020. Britain on Tuesday raised its terrorism threat level from "substantial" to "severe", meaning an attack is highly likely. (Photo by Ray Tang/Xinhua)