He was asked in an interview published Friday whether the "Leave campaign" was right to argue that Britain would be able to negotiate the same trade deal with the EU if it left than the one it currently enjoys as a member.
"I think that's a big mistake. The best possible trade deal is the one you have and especially the one just renegotiated with the rest of the EU," Macron said, Reuters reported.
Britons are set to vote in a crucial referendum on June 23 to decide whether to back a so-called Brexit - or exit from the European bloc.
UK Prime Minister David Cameron is leading the campaign to keep the country in the EU, but faces an uphill battle as many politicians, even from his own Conservative Party, are struggling against the bid.