The terror attack threat level in the UK has shifted from severe to critical, Prime Minister Theresa May said following a second emergency Cobra committee meeting. She added that armed forces will be dispatched to assist the police.

FOLLOW LIVE UPDATES: Manchester Arena bombing

"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," she said in a televised statement. “It is a possibility we can't ignore that there are a wider group of individuals linked to this attack.”

May said that she does not want the public to be unduly alarmed and called her move “a proportionate and appropriate response.”

She added that according to the assessment of the UK security services, another “attack remains [not only] highly likely but that a further attack may be imminent."

The last two occasions the threat level was raised to Critical the alert lasted no more than a few days: pic.twitter.com/GpzWJF4jMX — Alan Travis (@alantravis40) May 23, 2017

“We stand defiant... the terrorists will never win, and we will prevail,” May said.

The prime minister also praised the emergency services response to Monday’s terrorist attack in Manchester by saying that “evil can be overcome by good.”

Expanding on the governments response, May said police officers, who are currently guarding public offices, will be temporarily replaced by British Armed Forces personnel, adding that Operation Temperer was now in force.

The plan further envisages the deployment of up to 5,000 soldiers on the streets of British cities. Armed forces would also boost security at key locations and could also be deployed at public events such as concerts or sports competitions.

Assistant Commissioner Mark Rowley, head of National Counter Terrorism Policing called Operation Temperer "part of an agreed and well-rehearsed plan," under which "military personnel will remain under the command and control of the police service."

Briefing the public on the investigation which is making "good progress" with ongoing raids, Rowley said "at this stage it is still not possible to be certain if there was a wider group involved in the attack."

READ MORE: Manchester suicide bomber named by police

May’s statement came a day after the city of Manchester was rocked by a blast after a concert by US singer Ariana Grande at the Manchester Arena. The suicide bombing attack left 22 people dead, including an eight-year-old girl, and about 120 others injured.

The suspected bomber was identified by authorities on Tuesday as 22-year-old Salman Abedi following a raid on his apartment. According to authorities, the Manchester man of Libyan descent studied business at Salford University before dropping out.

READ MORE: Manchester police arrest suspect and carry out raids in terrorist attack probe (PHOTOS, VIDEOS)

During Tuesday's raid at Abedi's registered address, police detained a 23-year-old man who has not yet been named. Following his arrest, police carried out two separate raids on homes in the suburban areas of Manchester.

Greater Manchester Chief Constable Ian Hopkins told reporters that authorities are working to establish whether Abedi was working alone or as part of a wider network. The Islamic State has meanwhile claimed responsibility for the atrocity.