Greater Manchester Police said Thursday that they are responding to an incident at a local college in the northwest English city, with British troops and a bomb disposal unit supporting them, just days after a deadly terrorist at the conclusion of an Ariana Grande concert late Monday.

The use of troops to deal with security situations has been in place since Tuesday, when Prime Minister Theresa May raised the country's terrorist threat level to "critical", the highest of five stages, and cautioned that a new attack could be imminent.

"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.