The UK's terror threat level has been increased following the taxi explosion at the Liverpool Women's Hospital yesterday.
A government source has said the terrorism threat level has increased to severe meaning an attack is "highly likely".
It was previously at a threat level 'substantial' meaning an attack was "likely".
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There have been four arrests in connection with the blast that took place just before 10.59am on Sunday, November 14.
Assistant Chief Constable Russ Jackson, Counter Terrorism North West said: "Our enquiries have led us to two addresses.
"The first was Sutcliffe Street in the Kensington area of Liverpool and at this location three men aged 21, 26 and 29 were arrested yesterday under S41 of the Terrorism Act.
"A short while ago, again in the Kensington area, a further man aged 20 was arrested under S41 of the Terrorism Act."
Police believe they know the identity of the suspected bomber but can not confirm who it is at this time.
While it is being treated as a terror incident the motive remains unclear.
The Home Secretary said the attack had had a "very significant impact across the community" in Liverpool and her thoughts were with people in the city.
Priti Patel told broadcasters: "Alongside that, I want to give my personal thanks, and the thanks of the Government, to our emergency services, the police in particular. I've been in touch with the Chief Constable, other emergency service workers as well, and our armed forces who've been part of the operational work that's on the ground.
"Now, of course, that means we continue to work with our world-class security, intelligence and policing services.
"There's a live investigation taking place right now. They will need the time, the space, to do the work that they are doing in terms of investigating the incident.
"But of course, we as a Government, I as Home Secretary, continue to work with everyone when it comes to the security of our country and making sure that we're taking all the necessary steps required."
Priti Patel said the "context" of there being two terrorist incidents in the space of a month was "really important".
The Home Secretary told broadcasters she could not expand "for obvious reasons", but added: "I should just conclude my remarks by saying that our security and intelligence services prevent all sorts of acts, day in, day out.
"And of course, they understand the landscape, they see context, they see all sorts of things that keep our country safe and secure every single day and that work will continue."
There are five threat levels set by MI5's Joint Terrorism Analysis Centre and the Security Service.
There are 5 levels of threat:
- low - an attack is highly unlikely
- moderate - an attack is possible but not likely
- substantial - an attack is likely
- severe - an attack is highly likely
- critical - an attack is highly likely in the near future
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