In a statement in Downing Street, May said it was "now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack" that targeted "defenceless young people".
The Prime Minister said the attack, which also left at least 59 people injured, was the result of "cold calculation".
She said the suicide bomber deliberately chose the place where he could cause "maximum carnage" when he detonated a bomb at a pop concert in Manchester.
The British Prime Minister made her statement at the steps of Downing Street in London soon after she chaired a meeting of senior ministers and intelligence officials at an emergency response meeting of the Cabinet Office Briefing Room (COBRA).
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Describing the attack at Manchester Arena in the heart of the city as an act of "sickening cowardice", she confirmed that the identity of the attacker is now known to the police but will not be released while investigations are ongoing.
"It is now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack. It is the worst attack Manchester has suffered, and the worst to hit the north of England," Maysaid.
Her statement came as reports emerged of an emergency evacuation of a shopping centre near the site of last night's explosion at the Ariana Grande concert.
The Arndale Shopping Centre, the site of an Irish Republican Army (IRA) bombing from June 1996, was soon cleared for reopening to shoppers.
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