Paris (AFP) - 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 is the future of the European Union... that is at stake," Hollande said on Wednesday, warning that a Brexit would put Britain at "very serious risk" of losing access to the bloc's single market.

Hollande was speaking at a joint news conference with Slovakian Prime Minister Robert Fico, whose country will take over the EU's rotating presidency on July 1.

"It's more than the future of the United Kingdom that is at stake, it's the future of the European Union," Hollande said.

A Brexit would "necessarily have extremely serious consequences," he added.

France would consider such a decision "as the United Kingdom's departure from the European Union, and this would be irreversible," Hollande said.