The UK’s terrorism threat level has been upgraded from “substantial” to “severe”.
The move means security chiefs believe that an attack is highly likely but there is no specific intelligence of an imminent incident.
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 Paris last month.
Home Secretary Priti Patel said the British people should be “alert but not alarmed”.
“This is a precautionary measure following the horrific events of the last week in France and last night in Austria and is not based on a specific threat.”
She added that significant steps have already been taken to amend powers and strengthen the tools for dealing with developing terrorist threats.