Darmanin told French radio station RTL this morning that agents at the border would act in many cases as if Brexit has already happened, to ensure the service will be ready by 31 October.

He said: “For a month, we will behave as if there were Brexit for a large number of companies, so we will set up a kind of dress rehearsal for us to be all-in – ready by the end of October.”

Darmanin will go to Calais today to oversee permenant new facilities on France’s border with the UK, set up specifically to cope with trade after Brexit. The extra capacity at the ports includes 700 newly recruited additional customs officers.

About 100,000 French companies import or export to or from the UK, Darmanin said, and “Brexit, whether hard or soft, and even more so if it is hard obviously, will be difficult for the European economy and the French economy.”

He said: “We will still trade with Great Britain, but according to the rules of the World Trade Organisation.

“It will be like a link with South Africa on our doorstep. We trade with South Africa. It’s just more administrative than when it’s Belgium or Spain.”

Darmanin tweeted: “Our priority is to ensure the fluidity of trade. Our duty is to prepare our country for all scenarios.

“That’s what I have done, as customs minister, to be ready on D-Day to ensure the smooth flow of trade, including after a no-deal Brexit.”

The minister’s announcement has come on the same day that the UK government promised £30m would be spent on ensuring the smooth flow of trade at British ports.

It will pay for upgrades to port infrastructure, road and rail links, and some will go towards “building resilience within local government”.