The UK’s terror threat level will be raised from “substantial” to “severe”, meaning an attack is now judged to be “highly likely”, Sky News understands.
The move follows Monday night’s attack in Austrian capital Vienna, in which four people were killed.
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.
The terror threat level had been at “substantial” since November last year, when it was lowered from “severe” for the first time in five years.
The “severe” level is the second-highest level, with only “critical” above it – which was reached in May 2017 after the Manchester Arena bombing.
Following Monday night’s attack in Vienna, Prime Minister Boris Johnson said he was “deeply shocked”
He added on Twitter: “The UK’s thoughts are with the people of Austria – we stand united with you against terror.”
The five terror threat levels are:
Low – an attack is highly unlikely
Moderate – an attack is possible, but not likely
Substantial – an attack is likely
Severe – an attack is highly likely
Critical – an attack is highly likely in the near future