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.
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
The decision to raise the UK terror threat level is a sobering indication of just how concerned intelligence officials are about the possibility that a wave of terror attacks across Europe could spread to Britain.
That decision is taken independently of government, by JTAC made of members of the intelligence services and other officials.
The threat level now stands at ‘severe’, meaning a terror attack is highly likely.
It was reduced to ‘substantial’ in November 2019, after intelligence analysts assessed the threat to the UK from international terrorism was still significant but had reduced enough to suggest planned attacks were no longer highly likely.
Now, the ongoing threat facing European nations has been rapidly reassessed following multiple attacks against French interests and Monday’s marauding gun attack in the Austrian capital.
The motivation for the latest attack is still not entirely clear, but the gunman, who was shot dead by police, has sworn allegiance to the Islamic State terror group.
The group has already called for attacks against French interests because the Paris government’s robust defence of the right to publish caricatures of the prophet Muhammad.
The stance has angered many in the Muslim world, who see such depictions of their most revered prophet as grossly offensive.