"I am appalled to hear the news from Nice this morning of a barbaric attack at the Notre-Dame Basilica," Johnson said on Twitter in both English and French.

"Our thoughts are with the victims and their families, and the UK stands steadfastly with France against terror and intolerance."

British Foreign Secretary Dominic Raab added in a separate statement that Britain offered "every support to the French people in pursuing those responsible for this appalling attack".