British Prime Minister Theresa May says the United Kingdom is raising the threat level to critical, meaning an attack may be imminent.
May says Salman Abadi, the suicide bomber who killed 22 people at a concert in Manchester, may have been part of a bigger network.
That also allows police to call in the military to guard certain key posts. Armed police officers will then be able to better patrol key areas.
She said Abadi was born and raised in Britain.
The level previously stood at the second-highest rung of ``severe'' for several years.
May said critical status means armed soldiers may be deployed instead of police at public events such as sports matches.
The move comes less than 24 hours after 22 people were killed and dozens injured when a bomb exploded at an Ariana Grande concert at Manchester Arena.