Prime Minister Theresa May says Britain's threat level from terrorism has been raised 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.
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.
Investigators are continuing to sweep across the city of Manchester, on the hunt for any possible accomplices to the deadly attack on Ariana Grande concert goers.
British police say they have raided two sites in the northern English city and arrested a 23-year-old man at a third location.
Police identified the bombing suspect as 22-year-old Salman Abedi.
Thousands of people from all walks of life turned out today for a vigil in Manchester's Albert Square to honour the 22 victims and 59 wounded.
An 8 year old girl has been identified as one of the victims.
Meanwhile, Canada's Defence Minister Harjit Sajjan told reporters in Washington today that Canada's security agencies are continuing to learn from events like these and working to prevent future attacks.