The French ambassador, Sylvie Bermann, has told Welsh business leaders the UK remaining in the European Union is in everyone's best interests.
At a speech in Cardiff, she warned leaving would harm the UK, France and the EU.
She said Wales exported almost £650m of goods and services to France annually.
Supporters of the referendum campaign to leave argue trade would continue without EU membership and the UK would be in a stronger position.
Ms Bermann was speaking at the Cardiff Business Club with leaders, including the Confederation of British Industry and the Federation of Small Businesses, in attendance.
"Our president has been clear that France wants the UK to stay within the EU; it's in the interests of UK, of France and the EU as well," she said.
Nick Pearce from Object Matrix, a Cardiff company whose software is used by French companies such as EDF and France Televisions, was also at the speech.
He has seen his business with France grow from £15,000 a year to £800,000 annually since 2009.
Mr Pearce said links between the two countries had been vital in helping to grow the business, which employs 20 people.
However, others who export to the EU are increasingly worried about the impact of regulation.
Biopharm Leeches, based near Llanelli, Carmarthenshire, breeds and sells medicinal leeches to hospitals around the world.
Bethany Sawyer from the company is concerned about the future of her firm should the UK remain within the European Union.