Jean-Marc Puissesseau, the head of the port of Calais said if the Brexit deal results in a hard border that involves mandatory customs and sanitary checks at the ferry terminal then there will be traffic chaos on all roads leading to the town.

At a private meeting in the Brussels Puissesseau said UK Prime Minister Theresa May and Michel Barnier needed to put plans in place to ensure there is no congestion at Dover or Calais after Britain's eventual exit from the EU, the Guardian newspaper reported.

“I am worried about the slowdown of traffic if there are controls. Imagine 2m trucks being checked in Calais,” he said.

The Calais port chief lamented the future problems he expects at the port as suppliers from the UK try to pass their goods through EU controls.

“The UK is part of the 21st century. But this takes us back 100 years. This is sad,” he said. “From Brexit day, 100 percent of our traffic will be from outside the EU. I tell you honestly that GB will be a third country, this frightens me. There’s such a long history between the UK and EU.”

He also suggested Britain could be hit by food shortages if there are problems with suppliers from the EU getting across the Channel.

“At the moment, 70 percent of food imported comes from the EU. Even if that goes down to 50 percent after Brexit because of controls, it still needs to flow smoothly; people still need to eat,” he said. “If there are delays it could end up rotting on the side of the road.”

The port chief expressed regret that Brexit also threatens the viability of Calais' three-year €700 million expansion plan which includes new docks to accommodate wider and longer ferries.

“We based our calculation on the growing population in the UK. We thought if there were more and more people, then their needs for food, cars, everything, increases and traffic will increase and we need to prepare for that,” said Puissesseau. “But then Brexit comes along and we have a new problem.”

Xavier Bertrand, the president of the region of Hauts-de-France, which includes the ports of Calais, Dunkirk and Boulogne-sur-Mer, also expressed concern about the possible future chaos around the northern port.

Bertrand suggested the problems at Calais could be 10 times worse than at the Irish border.

“I know Ireland is going to be a real problem, but please remember the economic issues in Ireland are 10 times smaller than what is going to happen here,” he said. “This is a black scenario, but it is going to get darker and darker,” he said.

Bertrand and other politicians in northern France have repeatedly expressed concern that Brexit could damage the economy of a region that is already hit by some of France's highest unemployment rates.

Earlier this year President Emmanuel Macron tried to reassure politicians and business leaders in the area that France will fight to ensure the territory remains attractive.

“I realise how much uncertainty there is in several economic sectors; fishing, industry, logistics,” Macron said in a speech in Calais, ahead of a meeting with Britain's Prime Minister Theresa May in January.

A so-called 'hard border' with Britain could crimp the billions of euros' worth of goods that flow through the port each year, a grim prospect for local businesses and industry executives.

“No matter the changes to come and the relations between the European Union and the United Kingdom, the territory will remain attractive in these areas,” he said.

He said France would press its concerns with the EU's chief Brexit negotiator Michel Barnier, who is scheduled to begin talks on an eventual trade deal with Britain in March.

“The region's interests will be fully taken into account in the negotiations that France will lead, and I will make our case known in March with our negotiator, Mr Barnier,” he said.