The raised national threat level means that a terror attack in the UK is “highly likely”.
It comes after the latest terrorism incident on Sunday in Liverpool and others in recent weeks.
The decision was taken by the Joint Terrorism Analysis Centre (JTAC) along with the Security Service (MI5).
The UK was last at ‘severe’ in November 2020 before being lowered to substantial in February 2021.
Deputy Chief Constable of Lincolnshire Police, Jason Harwin said: “It is vital that we remain alert and not alarmed as this National Threat Level is raised. The recent events that have taken place is a stark reminder that we must not be complacent, we must remain vigilant.
“We encourage the public to continue to report suspicious activity and urge anyone with information to come forward. We are monitoring the situation closely and will respond appropriately should any specific threat be identified.”
Threat levels are designed to give a broad indication of the likelihood of a terrorist attack.
LOW means an attack is unlikely.
MODERATE means an attack is possible, but not likely.
SUBSTANTIAL means an attack is a strong possibility.
SEVERE means an attack is highly likely.
CRITICAL means an attack is expected imminently.
Threat levels do not have an expiry date. They can change at any time as different information becomes available to security agents.
Police have released the name of the man they believe was involved with a terrorist attack which took place in Liverpool.
Emad Al Swealmeen, 32, has been named by Counter Terrorism Police North West as the man they “strongly believe” was involved in a suicide bombing which saw a taxi exploded in flames outside Liverpool’s Women’s Hospital shortly before 11am on Sunday (November 11).
Police have since raided two addresses connected to Al Swealmeen, where they say “significant items” were recovered.
Russ Jackson, head of Counter Terrorism Policing North West, said that the passenger, who they believe to be Al Swealmeen, appeared to have made an improvised explosive device which caused the explosion.
The incident left the passenger dead and the driver injured.
Taxi driver David Perry was named as the driver in question, and has since been declared a “hero” by Prime Minister Boris Johnson and Liverpool Mayor Joanne Anderson.
In footage from the incident, Parry had appeared to quickly lock the attacker in his car before the device exploded.