The UK's terror threat level was raised from "substantial" to "severe" on Tuesday, the second-highest alert level which means an attack is now judged to be "highly likely". UK Home Secretary Priti Patel described the action as a "precautionary measure" following terror attacks in France last week and in Austria this week.
"The British public should be alert but not alarmed," said Patel, as she confirmed there would now be "more visible policing across the country". "That is right in light of the threat going up... the public should not be alarmed this is a precautionary measure," she said.
"As I have said before, we face a real and serious threat in the UK from terrorism. I would ask the public to remain vigilant and to report any suspicious activity to the police," she said.
The move follows Monday night's shooting in Vienna in which four people died. Last week, three others died in a knife attack in Nice, France, and a teacher was murdered in the French capital Paris last month.
Assessments of terror threat levels are taken by the Joint Terrorism Analysis Centre (JTAC), part of the MI5 intelligence service, which makes its recommendations independently from the government. They range between five categories of low, moderate, substantial, severe and critical.
The UK's terrorism threat level was raised to the highest rating, "critical", in the days following the Manchester Arena bombing in May 2017. The threat level remained at the second highest rating, "severe", until last November last year when it was downgraded to "substantial", where it has stayed until now.
