Almost 1,000 soldiers will be deployed in Britain as part of Operation Temperer, but none will be in Northern Ireland.
Home Secretary Amber Rudd raised the threat level facing the UK to critical on Tuesday night for the first time in a decade after a suicide bomber killed 22 people and injured scores more at a pop concert at the Manchester Arena.
A total of four people have now been arrested by detectives investigating the bombing with searches continuing into the blast on Monday night.
Ms Rudd has confirmed that 984 military personnel will be providing extra security as they take up positions around the country.
It is understood that soldiers to replace police at sensitive security points under Operation Temperer, which was originally devised in 2015.
In Northern Ireland there will be an additional police presence.
TUV leader Jim Allister has questioned why Northern Ireland will not be included in the operation.
"While across Great Britain the military are being deployed to protect key sites following Monday night's horrific attack in Manchester I note that this will not apply in Northern Ireland," he said.
"This is in spite of the fact that there will be an increased police presence at some sites, on the transport network and in places where there are large crowds.
"Given that it was felt necessary to deploy the military in other parts of the UK I would like to understand the logic behind why this is not happening in Northern Ireland.
"The decision to deploy more police officers suggests that there is a belief that there is an increased threat to public safety in our Province.
"It would be outrageous if political considerations and a desire not to offend Republicans compromised public safety."
Troops were deployed across Northern Ireland during the Troubles as part of Operation Banner. This saw around 21,000 troops serve here since the 1970s. That operation ended in 2007 following peace agreements.
However some army presence remains in Northern Ireland with military bomb experts available to examine suspect devices.