British Prime Minister Theresa May raised the terrorism threat level from “severe” to “critical” on Tuesday night, saying another attack on the United Kingdom may be “imminent.”

She made the announcement at approximately 9:45 p.m. London time.

The warning comes less than 24 hours after a suicide bomber killed 22 people and injured 59 others at an Ariana Grande concert in Manchester, England.

“It is a possibility we cannot ignore, that there is a wider group of individuals linked to this attack,” May said, warning of a follow-up incident.

May said critical status means armed soldiers may be deployed instead of police at public events such as sporting events.

“The change in the threat level means there will be additional support available to the police as they work to keep us safe,” she added. “Operation Temperer is now in force.”

Take That and Blondie are among the acts that postponed their U.K. shows following the terrorist attack.

“I do not want the public to feel unduly alarmed,” May said during the press conference. “We have faced a serious terrorist threat in our country for many years. The operational response I have just outlined is a proportionate and sensible response to the threat our security experts judge we face.”

“I ask everybody to be vigilant and to cooperate with and support the police as they go about their important work,” she added.