The measure is part of a “well-established plan” known as Operation Temperer, which could see up to 5,000 armed soldiers working with the police.
Today, home secretary Amber Rudd confirmed that 984 military personnel will be providing extra security as they take up positions around the country.
The Home Office declined to comment on the cost of this.
The prime minister Theresa May announced the measure yesterday, following the second meeting of the emergency Cobra committee in the wake of the suicide bombing in Manchester which killed 22 and injured more than a hundred on Monday evening.
The Greater Manchester Health and Social Care Partnership has confirmed today that 119 people were taken by ambulance or made their own way to hospital following the bombing.
Currently 64 are still being treated and of those, 20 are still in critical care across Greater Manchester.
The attacker has been named as 22-year-old Mancunian of Libyan descent, Salman Ramadan Abedi.
Speaking outside Downing Street, Theresa May, said that the independent Joint Terrorism Analysis Centre had called for an increase in the terrorism threat level for the time being, from severe to critical.
“This means that their assessment is not only that an attack remains highly likely, but that a further attack may be imminent,” she said.
She added: “The change in the threat level means that there will be additional resources and support made available to the police as they work to keep us all safe.”
She confirmed that the police have asked for authorisation from the secretary of state for defence to deploy a number of armed military personnel in support of their armed officers – a request that has been approved.
May said: “This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations.”
Political campaigning for the general election remains suspended.
