THERESA May has condemned the "horrifying" terror attack that killed at least 84 people, including around a dozen children, when a terrorist drove a truck through crowds celebrating Bastille Day in Nice.

The new Prime Minister said Britain must redouble its efforts to defeat "brutal" terrorist "murderers", and has called a meeting of senior officials in the Government's emergency Cobra committee.

France has declared three days of national mourning following the atrocity, which comes after attacks in November in Paris in which 130 died and in January 2015 that killed 17.

Eyewitnesses said the Nice attacker - reported to be 31-year-old Mohamed Lahouaiej Bouhlel, a man of French-Tunisian origin who was known to the authorities for anti-social behaviour - swerved from side to side to kill as many people as possible as he drove for a mile along the Promenade des Anglais on the seafront of the city on the French Riviera.

He is said to have pulled a gun from the cab as part of the premeditated attack before being shot dead by police, with people fleeing into the sea in a bid to escape.

New Foreign Secretary Boris Johnson said a Briton is believed to be among the scores injured.

Speaking to the BBC as he left his home, Mr Johnson, who celebrated Bastille Day at the French Embassy in London, said: "It's an absolutely appalling incident and there will be ministerial meetings later on today to discuss the implications.

"Clearly this represents a continuing threat. If this is a terrorist incident, as this appears to be, this represents a continuing threat to us in the whole of Europe and we must meet it together.

"The only information that I have is that there is one UK national who is injured."

Speaking ahead of a visit to Scotland, Mrs May said she will speak to French president Francois Hollande today and "make clear that the United Kingdom stands shoulder-to-shoulder with France today as we have done so often in the past".

She said: "If, as we fear, this was a terrorist attack, then we must redouble our efforts to defeat these brutal murderers who want to destroy our way of life.

"We must work with France and our partners around the world to stand up for our values and for our freedom."