Britain on Tuesday (3) upgraded the country's terrorism threat level from "substantial" to "severe", after a deadly shooting rampage in Vienna and several attacks across France.
"Severe" -- the second-highest of five levels -- means an attack is "highly likely", said the domestic intelligence service MI5, which announced the change on its website.
The threat had been deemed "substantial", where an attack is "likely", since November 4 last year.
Home secretary Priti Patel said: "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," she wrote on Twitter.
Monday evening's attack in Vienna left four people dead and came on the eve of a month-long coronavirus lockdown across Austria. England enters a four-week shutdown on Thursday.
Police in the Austrian capital said the attack was carried out by a known Islamist extremist who had spent time in prison.
Three people were killed in a knife rampage in the southern French city of Nice last week in an attack the government has described as an act of "Islamist terror".
It followed the shock beheading by a suspected Islamist radical of a teacher who showed a class a cartoon of the prophet Mohammed.
The UK decision was made by the Joint Terrorism Analysis Centre (JTAC), which conducts a formal review of the threat level every six months, independent of government ministers.
JTAC's membership includes the security services MI5 and its overseas counterpart MI6, and police, and assesses all intelligence relating to terrorism at home and abroad.
The level was briefly raised on two occasions to "critical", the maximum level indicating an attack is "highly likely in the near future", in May and September 2017.
The move was in response to conflicts in Syria and Iraq. Manchester and London also both suffered deadly Islamist militant attacks that year.
It was also changed to "critical" in August 2006 after the security services said they had foiled a plot to blow up flights between Britain and the United States.
