Britain has warned that a terrorist attack is now highly likely as it raised the official threat assessment to severe in the middle of ISIS assaults around Europe. Terrorist incidents in France and Monday night's attack in the Austrian capital Vienna, in which four people were killed, pushed officials into a heightened state of readiness. Last week, three people died after a knife attack in the French city of Nice, while teacher Samuel Paty was murdered in Paris last month. The decision to raise the UK's threat level has been taken by the Joint Terrorism Analysis Centre, the government body that assesses the level of risk to the public. The centre is based at MI5's headquarters in London and is made up of counter-terrorism experts from the police, government and security agencies. The terror threat level had been at "substantial" since November last year, when it was lowered from "severe" for the first time in five years. The severe level is the second-highest level, with only "critical" above it. Critical was reached in May 2017 after the Manchester Arena bombing. Prime Minister Boris Johnson said he was "deeply shocked" after Monday night's attack in Vienna. "The UK's thoughts are with the people of Austria," Mr Johnson said. "We stand united with you against terror." Neil Basu, the head of counter-terrorism for the Metropolitan Police, said the UK had made its own assessment of the risk without any indication of a direct link. “Our thoughts are with all of the victims, their families and loved ones of the recent attacks both in Austria and France, as well as all of the emergency service personnel who continue to respond to and investigate these atrocities," Mr Basu said. “At this time, there is no intelligence to link any of these attacks to the UK, and officers from Counter Terrorism Policing continue to work closely with our international partners and will, of course, provide any assistance where we can. "Today, the threat level from terrorism has changed from substantial to severe as a precautionary measure in response to the events in France and Austria. "This ... means it is highly likely that a terrorist attack could happen in the UK, but I want to stress that the change is not based on a specific threat. "However I do urge the public to remain vigilant and to report any suspicious activity to police. "Now, more than ever, we need communities to stand together and reject those who seek to sow division and hatred between us." Britain maintains a five-tier system of security alert: Low – an attack is highly unlikely;<br/> Moderate – an attack is possible but not likely;<br/> Substantial – an attack is likely;<br/> Severe – an attack is highly likely; and<br/> Critical – an attack is highly likely in the near future.