Boris Johnson launched into a rant in French to condemn claims by a French government minister that the so-called "Jungle" migrant camp in Calais could move to Britain if the UK leaves the EU.
The mayor of London told ITV News: "Donnez-moi un break" (Give me a break).
Switching to English, he added: "It's complete nonsense, it's completely untrue because after all this is not an EU agreement this is an agreement between Britain and France at inter-governmental level.
"There is no reason why it should be affected. This is all part of a great project to try to terrify people into wanting to stay in the EU."
Asked if he thought it would affect Britain's relationship with France, Boris again replied in French:
Reverting back to English, Boris then added: "I refuse to accept that we in any way going to imperil our relationships."
His comments come after French finance minister Emmanuel Macron said a so-called Brexit could end an agreement between Britain and France that allows UK border officials to conduct checks on the French side of the border.
Brexit campaigners have rubbished the claims, saying there was "no more chance of shifting Calais camps to UK than a refugee camp springing up outside of Terminal 5 at Heathrow."