The change in security level was made after major attacks occured in France and Vienna.

Britain's Home Office tweeted: "The independent Joint Terrorism Analysis Centre has changed the UK national terrorism threat level to SEVERE. This means that a terrorist attack is highly likely. People should be alert but not alarmed & report concerns to the police."

Interior minister Priti Patel tweeted: "The Joint Terrorism Analysis Centre has changed the UK terror threat level from substantial to severe.This is a precautionary measure and is not based on any specific threat. The public should continue to remain vigilant and report any suspicious activity to the police."

Vienna was shocked as the gunman killed four people during the attack which has been widely condemned.

The man, described as a 20-year-old Islamist terrorist who had been freed from jail last December, was shot dead by police as he fired on passers-by, reports BBC.