World leaders have united in condemning the Manchester Arena terror attack.
US First Lady Melania Trump wrote on Twitter: “My thoughts and prayers to the families of #Manchester.”
French president Emmanuel Macron will speak to Mrs May about the attack later.
In a statement, his office said: “It’s with dismay that Mr Macron learned of the murderous attack which struck last night during a concert at the Manchester Arena.
“He expresses to the British people all the compassion and care of France which is at their side in mourning, with a particular thought for the victims and their families.”
French prime minister Edouard Philippe said: “The most cowardly form of terrorism has struck once more, targeting, as it did in Paris more than a year ago, a concert venue, aimed specifically and knowingly at very young girls gathered together for a moment of celebration and joy.
“In the wake of this abominable crime, I would like to express my sadness, the solidarity of the French people and unwavering friendship to Mancunians and all British people.”