U.K. Prime Minister Theresa May said Tuesday that she has raised the country's terrorist threat level to "critical", the highest of five stages, and cautioned that a new attack could be imminent.

Speaking outside Downing Street after a meeting with domestic security services, May said that investigations into the man alleged to have perpetrated the deadly attack on the Manchester Arena late Monday -- 22-year old Salman Abedi -- suggest he could have connections to terrorist operatives.

"It is now concluded, on the basis of today's investigations, that the threat level should be increased, for the time being, from severe to critical," May said. "This means that (the assessment of the security services) is not only that an attack remains highly likely, but that a further attack may be imminent."

By raising the threat level to "critical" from "severe," May said, the government can deploy up to 5,000 troops onto British streets at public events, such as sports competitions, although the military personnel would come under police command. The protocol is being called 'Operation Temperer', 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," May said.

"You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe," she added.

The threat increase marks the third time Britain has reached the fifth of five levels, and the first since 2007 when a man drove a Jeep Cherokee loaded with explosives into the Glasgow International Airport in Scotland. 

The decision follows Britain's deadliest terrorist incident since July 2005, when Muslim extremists killed 52 people with three separate suicide bombs across London's transport system at the height of the morning rush-hour commute.

London, and indeed the whole of the United Kingdom, remains on high alert after last night's attack at the conclusion of an Ariana Grande concert in Manchester Arena which left 22 people -- including an 8-year old child -- dead and 60 others wounded.

The attacker used a self-detonating explosive in the 21,000-seat capacity arena, police said, and noted that inquiries at this stage are focused on whether he acted alone or with the assistance of others.

"We have been treading this as a terrorist incident and we believe, at this stage, the attack was conducted by one man," Greater Manchester Police Chief Constable Ian Hopkins said in a statement. "The priority is to establish whether he was acting along or part of a network."