Britain has increased its security threat level to "critical" from "severe" following a suicide attack in Manchester that killed 22 people, including children, Prime Minister Theresa May said on Tuesday.

May also said members of the armed forces would boost security at key sites and military personnel might be deployed at public events such as concerts and sports events.

May said the independent body which sets the threat level had recommended it be raised after a man named by police as Salman Abedi set off an improvised bomb on Monday night as crowds streamed out of a concert.

"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 following a meeting of the government's crisis response committee.

"This means that their assessment is not only that an attack remains highly likely but that a further attack may be imminent."