In a 28-page advice booklet, the government said firms using British suppliers or sub-contractors should already be looking for alternatives.
And companies in specialist sectors operating under EU rules, such as pharmaceutical firms, were told they should consider moving their British operations back onto the European continent.
France's European Minister Nathalie Loiseau urged businesses “not to panic but to prepare” ahead of a possible no-deal Brexit on Friday.
“Keep calm and carry on,” she said, using the British wartime adage, adding that with the current Brexit D-Day set to take place on March 29th, businesses needed to act now.
France, like other EU countries, is bracing for a potentially calamitous British exit on March 29 after the parliament in London resoundingly rejected a deal negotiated by Prime Minister Theresa May.
Junior Finance Minister Agnes Pannier-Runacher said France was “hoping for the best but planning for the worst”.
“There is now a significant risk that Britain will leave the EU without a deal and there is no time to lose for French businesses that work with the UK or employ British citizens,” Pannier-Runacher said.
“That means that from March 30th Britain counts as a third country and all our deals with them agreed under the European Union will no longer be included.
“Businesses have to prepare and take the necessary measures, something in which professional organisations will have to play a key role,” she said.
But she also added that there “was no reason it should be difficult”.
As one of Britain's closest neighbours, France has been taking such a prospect seriously, activating a “no deal” plan that unlocks up to 50 million euros ($57 million) for bolstering security at ports and airports.
It has begun recruiting an additional 740 customs officials and veterinary inspectors, while passing legislation that allows for emergency decrees in the
event of a “no deal”.
France's European Minister Nathalie Loiseau. Photo: AFP
With just 63 days to go until Britain's scheduled exit, Pannier-Runacher warned that a no deal would fling France “into an unprecedented situation with a major trading partner”.
The advice warns French companies with staff in Britain to work out how it will affect matters such as social security contributions, and to possibly revert to using temporary workers.
Firms should consider transferring financial services contracts to EU countries, and withdrawing confidential data held within Britain.
And French companies working alongside British partners on EU-funded projects should now be looking elsewhere, the advice says.
Loiseau said that while France would seek a post-Brexit relationship with Britain that was “close and mutually beneficial”, it would inevitably be a relationship that has been “downgraded”.
“There is no relationship more simple, more profitable, more complete — between businesses, between citizens — than being a member of the European Union,” she said.
French officials are planning to hold around 30 meetings around the country to help local businesses deal with the Brexit fallout.
Some 30,000 French companies currently export to Britain — tariff-free as part of the EU's customs union.
These exports make up around three percent of France's annual output.