Authorities in the northern Hauts-de-France region have demanded President Emmanuel Macron break ranks with Brussels to defend their interests.
The region is a key area for trade with the UK, comprising Calais, France’s largest passenger port, Boulogne-Sir-Mer, a huge fishing port, and Dunkirk, its third-biggest cargo hub.
Any disruption of trade with the UK could decimate the regional economy, its leaders fear.
Nervous pundits have often predicted a hard Brexit would harm the UK above all, however in Hauts-de-France local authorities say they face even bigger fears.
The president of Hauts-de-France, Xavier Bertrand, is now urging Mr Macron to take up his cause.
Mr Bertrand told the Financial Times: “I am pushing the government… The divorce is settled, we must go full steam ahead.”
The former Labour government minister said now was the time to exert influence over trade negotiations.
He said: “Let’s not be naive. I don’t want my region to be outpaced by others.”
Mr Bertrand added that a punitive Brexit deal would not only hurt Britain, but France too.
He said: “We shouldn’t seek to punish the British. I don’t want post-Brexit decisions that hurt cross-border regions.”
In particular, Hauts-de-France fishermen from Boulogne-sur-Mer will need access to UK waters to keep up their trade, Mr Bertrand said.
They currently rely on fishing in the UK for the bulk of their catch – and may not be able to make this up elsewhere.
There are also fears Dunkirk could be sidelined as companies opt for British ports over France once borders are closed.
Meanwhile businesses owners in Eymet, in the south of France, say their livelihoods are at stake in the case of a bad Brexit deal.
Chiefs at the picturesque village, nicknamed “Little England” due to the large numbers of British expats living there, say local businesses could go into meltdown if Britain leaves the EU without a trade deal.
Around 300 Britons are based in the medieval village in the Dordogne region, which also boasts an English pub, tea room and shop.
More than a third of the village’s business comes from Britons – and this could be in doubt without a trade deal.
One market stall owner said: “If the British were not here, this would be a tiny market.
“The British represent 35 per cent of the clientele in the winter months when business drops.”
Cheesemonger Philippe Barb echoed this, as he said: “If there are no English here, we’re screwed! They’re a big part of our local economy.”
Locals said the French government would have to push for a good deal to protect citizens’ welfare.