Following a taxi explosion outside a Liverpool hospital that killed a man, the UK government today raised the level of terrorist threat in the country to "severe," which means there is a highly likely attack.

Ross Jackson, Chief Constable for Counter-Terrorism North West England, said that Sunday's explosion at Liverpool Women's Hospital resulted from an "explosive device" that a passenger put in the car.

For her part, Home Minister Priti Patel told reporters that the increase in the threat level came in the context of being the second terrorist incident in a month, and did not go into detail.

Patel added: "Our security and intelligence services prevent all kinds of actions, day after day, and of course, they see the context, they see all kinds of things that keep our country safe and secure every day, and this work will continue."

Three men in their 20s were reportedly arrested elsewhere in the city under the Terrorism Act on Sunday, and a fourth was detained on Monday, all of whom police said were believed to be associates of the murdered passenger.

British Prime Minister Boris Johnson is to chair a meeting of the Cobra Government Crisis Committee today in response to the explosion.

Doubts were raised about the motive for the explosion because of the timing - shortly before 11 a.m. in Remembrance Sunday, the moment when people across Britain are holding prayers in memory of the war dead.

Until today's announcement, Britain's official threat level was "real," the middle grade on a five-level scale. The Joint Terrorism Analysis Centre determines the threat level, based on intelligence on international terrorism at home and abroad.

The threat scale levels are low; It means that the attack is highly unlikely, moderate; It means that an attack is possible but unlikely, and real; Which means the attack is likely, and severe; It means that the attack is highly probable, and the final level is critical; It means that the attack is highly likely in the near future.