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The terrorism threat level has increased to severe meaning an attack is "highly likely", a Government source said.
Police have declared the blast from a car outside Liverpool Women's Hospital shortly before 11am on Sunday a terrorist attack but said the motive was unclear.
Detectives have arrested four people under terrorism laws, three aged 21, 26 and 29, over the explosion, which killed the suspect inside the taxi.
The decision to raise the threat level has been taken by the Joint Terrorism Analysis Centre (JTAC), who are based at MI5’s headquarters in London.
The last time the threat level was raised to severe was in November 2020 following a spate of terrorist attacks across Europe. Four people were shot dead by a gunman in Vienna while three others were killed in a knife attack in Nice.
In February 2021, this was downgraded to “substantial”, with home secretary Priti Patel saying it represented a “significant reduction” in the momentum of attacks across Europe.
Prime Minister Boris Johnson, speaking to a Downing Street press briefing, said “the British people will never be cowed by terrorism” following the attack in Liverpool.
Mr Johnson said the attack outside Liverpool Women's Hospital was "sickening".
The Prime Minister said: "What yesterday showed above all is that the British people will never be cowed by terrorism, we will never give in to those who seek to divide us with senseless acts of violence.
"And our freedoms and our way of life will always prevail."
He said emergency services responded with "such speed and professionalism".
But he added: "It is a stark reminder of the need for us all to remain utterly vigilant."
There are five terror threat levels, with only “Critical” higher than “Severe”. Authorities last announced a “critical” threat level following the Parsons Green train bombing in 2017.
Home Secretary Priti Patel said the terror threat level had been raised because there had been two attacks in the space of a month.
Speaking to broadcasters, she said: “The Joint Terrorism Analysis Centre, Jtac, are now increasing the United Kingdom’s threat level from substantial to severe.
“And there’s a reason for that, and that reason is because what we saw yesterday is the second incident in a month.”
Assistant Chief Constable (ACC) Russ Jackson, from Counter-Terrorism North West, said the passenger is likely to have brought an explosive device into the vehicle.
"We believe we know the identity of the passenger but cannot confirm this at this time,” he said.
The man’s motivation was "yet to be understood", Jackson added.