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 to see what Britain can do to help.
Speaking ahead of a visit to Scotland, the Prime Minister said: "I have asked my deputy national security adviser to chair a Cobra meeting of senior officials to review what we know and what we can do to help, and I will speak to President 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.
"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."
May said the Government was working to establish whether any British nationals were caught up in the attack.
"Our hearts go out to the French people and all those who have lost loved ones or been injured," she said. "While the full picture is still emerging, it seems that at least 80 people are feared dead and many others have been injured.
"It was innocent victims enjoying a national celebration with their friends and families. We are working urgently to establish whether any British nationals were caught up in the attack.
"Our ambassador is travelling to Nice today with consular staff and they will be doing all they can to help anyone affected."
London's French ambassador Sylvie Bermann spoke on the steps of the embassy in Knightsbridge, where people have begun leaving tributes to the victims.
She told reporters: "The 14th of July (Bastille Day) is a very symbolic day - it is a symbol of liberty, egality and fraternity and I think that is what the terrorists also want to fight."