British Prime Minister Theresa May said the United Kingdom has elevated the terror threat level from "severe" to "critical" as a result of Monday evening's attack in Manchester.

"This means their assessment that is not only an attack is highly likely but a further attack may be imminent," May told reporters at a press conference Tuesday.

The change was made at the recommendation of the Joint Terrorism Analysis Centre, under MI-5, and brings the threat to the highest possible level.

"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," May added.

The British secretary of defense has approved a request - under Operation Tempora - to deploy a number of armed military personnel to supplement armed officers in high-traffic areas of the country. Armed forces officers will report to police officers on site.

On Monday night, 22-year-old Salman Abedi carried a nail bomb into the Manchester Arena. The explosion killed 22 people and injured 59 others. May said police are still investigating whether Abedi acted alone.