Prime Minister Theresa May said this morning that the government is working "urgently" to establish whether any British nationals were caught up in last night's attack in the French city of Nice.
At least 84 people are dead after a lorry was driven into a crowd in a suspected terrorist attack during Bastille Day celebrations in the French Riveira.
"I am shocked and saddened by the horrifying attack in Nice last night," May said. "Our hearts go out to the French people, and to all those who have lost loved ones or been injured. While the full picture is still emerging, it seems that at least 80 people are feared dead and many others have been injured.
"These were innocent victims enjoying a national celebration with their friends and families."
She added that the British ambassador to France is travelling to Nice today to do "all they can to help anyone affected".
"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," May continued.
"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."
Boris Johnson said this morning that at least one Briton has been injured in a suspected terrorist attack in Nice.
"The only information I have is that there is one UK national who is injured but it is still too early to say at the moment," Johnson said.
French authorities have confirmed that at least 84 people were killed after a lorry was driven into a crowd in the city during Bastille Day celebrations last night.
At least 100 more were injured, with a further 18 people in critical condition, after the attack in the south of France.
French news agency AFP reported police had found identity papers belonging to a 31-year-old French-Tunisian man in the truck, alongside a gun.
The UK's new foreign secretary today described the attack as "appalling" and said it highlighted the continuing threat to Europe from terrorism. He is set to hold meetings later to discuss the implications the attack holds for Britain.