France would see Britain's exit from the European Union as "irreversible" if the Leave camp wins Thursday's referendum, President Francois Hollande said.
"It's more than the future of the United Kingdom that is at stake, it's the future of the European Union," Hollande said, before announcing a trip to Berlin next week ti discuss the fallout from the vote with German Chancellor Angela Merkel.
A Brexit would "necessarily have extremely serious consequences," he added warning that a Brexit would put Britain at "very serious risk" of losing access to the bloc's single market.
Hollande was speaking on the eve of the EU referendum in the UK, which will decide whether or not Britain remains a member of the European union.
French politicians have been eager to stay out of the campaigning up until now, not wanting to come across as interfering int British affairs, which may antagonise voters.
However Hollande's words of warning are echoes of previous cautionary tales offered up by himself and other members of the French government.
Earlier this year the president warned that there would be "consequences" for how migration was managed after meeting Prime Minister David Cameron for an Anglo-French summit in northern France.
"I don't want to scare you but to tell the truth, there will be consequences... including on the question of people... the way in which we manage migration issues," he told reporters.
Under the 2003 Le Touquet border treaty, Britain is allowed to carry out border checks on French soil, stopping many migrants.
Earlier that day France's Economy Minister Emmanuel Macron caused a storm when he suggested that the "Jungle" migrant camp would end up on the English side of the Channel if Britain voted to leave the UK.
Pro-Brexit campaigners have dismissed the statements from French politicians as pure scaremongering.
Elsewhere on Thursday the French were trying to influence the vote in a far more positive way by handing out love letters from the French to the UK at King's Cross Station in London.
However the original plan of Operation Croissant was to hand out the famous French pastries, however this was scuppered by British police who pointed to election rules that barred the handing out of food, in case it swayed voters minds.