As Britain's ruling Conservative Party prepares for a deeply divisive annual conference, opening in Birmingham on Sunday, concerns over a hard Brexit are mounting on the other side of the Channel.
In a study whose contents are revealed in Le Monde newspaper, the top French government representative in the Hauts-de-France department, Michel Lalande, warns that hundreds of jobs will have to be created to handle formalities at the border and queues of vehicles and people could "affect the handling of public order in the long term".
Some 32 million people and 4.2 million heavy goods vehicles pass through the Channel ports of Calais and Dunkirk and the Eurostar rail station in Lille every year.
If there is no Brexit deal, passport controls for people and customs checks for goods are set to become more complicated and time-consuming, overwhelming officials unless their number is greatly increased over a short space of time.
Queues for visa checks
Officials are drawing up a "specific plan for crisis management, in order to limit the effects of new procedures", Lalande tells the minister.
Given that Britain opted out of the Schengen no-borders agreement, passports are already checked at the border but the procedure will become more complicated if no agreement is reached, since British citizens will require visas to visit the European Union and EU citizens will need them to go to the UK.
That will mean travellers leaving vehicles, including coaches and lorries, and will probably mean border posts are overwhelmed, Lalande warns, adding that the length of time for passport control will double, leading to congestion at the Channel ports and in Lille.
Some 250 extra police will be needed, the study says, pointing out that they are unlikely to be available by 29 March, when Brexit should take effect.
Health checks on food
Transporting goods will also become more complicated.
Animal and vegetable products will have to undergo sanitary and phytosanitary controls in approved premises, which will take 18-24 months to build, according to the study.
Up to 970,000 shipments could need checking every year, requiring 195 new jobs to be created, it estimates.
Companies that are not used to exporting outside Europe will need to be informed of their rights and duties and contact established with the customs service.
Public Accounts Minister Gérald Darmanin has promised the creation of 700 customs officials' jobs over the course of the next four years and the government will appoint a Brexit coordinator in the next few days.
Toyota to pause production
Meanwhile in Britain, the UK boss of Toyota, Marvin Cooke, warned that production will stop for a time at the company's Burnaston plant in the event of a no-deal Brexit.
The plant, which employs 2,500 staff, produced nearly 150,000 cars last year, 90 percent for export to the European Union.
It relies on components from the EU for its "just-in-time" production of the Auris and the Avensis models.
"My view is that if Britain crashes out of the EU at the end of March we will see production stop in our factory," Cooke told the BBC, although he could not say whether it would be for how long.
Jaguar Land Rover said in July that it would cut costs in Britain in the event of a hard Brexit and in September BMW warned that it would close the factory where it makes the Mini for four weeks.