Francois Hollande has strongly criticised Britain's decision to quit the European Union and said the election of Donald Trump means it will be particularly costly.
Mr Hollande also said the UK will have to relinquish any of the bonuses which come with membership of the European Union after it leaves the bloc.
Although the Trump administration has said the UK will be at the front of a queue for a trade deal with the US, Mr Hollande suggested the new President's 'America First' foreign policy will make it more difficult for Theresa May to secure an agreement.
Speaking to The Guardian, Mr Hollande said “The UK’s problem is this: it had thought that in leaving Europe it would tie up a strategic partnership with the US. But it now happens that the US is closing itself off from the world.
"The UK has made a bad choice at a bad moment. I regret that.”
Theresa May has previously made it clear she will seek to establish a "bold free trade agreement" in negotiations with the EU, but Mr Hollande - speaking before a summit in Versailles which is likely to focus on the future of the EU without the UK - poured scorn on this possibility.
He maintained his position the UK can't have any of the perks of being an EU member-state without being in the Union, warning the country will become an "outsider."
Article 50, which will allow the United Kingdom to formally begin exit negotiations, is expected to be triggered in a matter of weeks in time with the Prime Minister's deadline of the end of March.