Britain on Tuesday upgraded the country's terrorism threat level from "substantial" to "severe", in the wake of Monday’s attack in Vienna and several attacks across France, AFP reports.
"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.
Interior minister 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 will enter a four-week shutdown on Thursday.
The Islamic State (ISIS) terrorist organization on Tuesday claimed responsibility for the deadly shooting attack in Vienna.
The announcement followed confirmation from authorities that one of the shooters was a 20-year-old man who had been previously convicted of seeking to travel to Syria and join ISIS but was released from prison early due to his young age.
In France, meanwhile, three people were killed in a knife attack in the southern French city of Nice last week.
It followed the beheading 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, according to AFP.
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.
Britain has been hit by several terrorist attacks in recent years. In February, a man stabbed and wounded three people in south London before being shot dead by police.
Three months earlier, three people were killed in a terrorist attack at London Bridge.
London Bridge was also the scene of a terrorist attack in June of 2017, in which Islamic State (ISIS)-inspired attackers ran down people on the bridge, killing two. They then proceeded to stab several people to death in nearby Borough Market.