LONDON — In March of last year, French customs agents highlighted the mayhem that Britain’s departure from the European Union was likely to cause for drivers and workers at ports, train stations and the tunnel that links Britain and France.
To illustrate the point, the agents took the time to check the identifications of drivers and vehicles traveling from Britain to France — a step that was not mandatory while the country was a member of the bloc — which led to a miles-long line of trucks and cars at the border. Yet the French authorities struck a reassuring tone, saying that there was plenty of time to find a solution, because Britain was expected to remain in the bloc’s single market and customs union until the end of 2020.
More than 18 months later, and with just over a month left before the deadline for a post-Brexit trade deal between Britain and the European Union, truck drivers on Tuesday again witnessed what may await them after Dec. 31.
Another dress rehearsal, this time by French border police officers testing post-Brexit immigration procedures, resulted in a five-mile traffic jam in Kent, in southern England, up to the entrance of the Eurotunnel, which connects the country to France.
As Prime Minister Boris Johnson of Britain faces mounting pressure to reach a trade deal in the final stage of negotiations with the European Union, officials on both sides of the English Channel have warned that crossing it will be slower starting on Jan. 1. (The French police operate checks on the British side of the tunnel and ports, and the British police do so on the French side.)
That was evident on Tuesday, as French border police officers checked the papers of trucks crossing from Britain into France over a nine-hour period, according to the French authorities and a spokesman for Getlink, which manages the tunnel. That led to traffic buildups until late in the afternoon.
The aim was to carry out a “stress test” of new immigration processes that might be required in the event of a no-deal Brexit on Jan. 1, said John Keefe, a spokesman for Getlink.
“If there’s no deal and the U.K. becomes a full third country without membership of the E.U., then you can expect full passport controls, a questionnaire for each traveler and questions about your destination, length of stay and proof of means,” Mr. Keefe said — “all the kind of classic third-country questions when you don’t have an easy crossing.”
The regional authorities in northern France said that the “full-scale exercise” was aimed at figuring out how to avoid traffic congestion on Jan. 1, and Mr. Keefe said the traffic jam had arisen in part because just two out of four lanes were open.
Mr. Johnson has repeatedly said that Britain is ready for a Brexit without a trade deal, and the European Union has said that changes at the border are inevitable, regardless of the negotiations’ outcome.
For truck drivers, that will mean having passports stamped, obtaining new licenses for access to both markets, and providing additional information like destinations and lengths of stay. They will also have to go through additional checks on products and may have to pay customs duties in both directions.
The anticipated procedures have alarmed unions and officials in Kent, which worry that the backlogs will cause serious traffic disruptions.
They have also warned that Kent could become “the toilet of England” as drivers and thousands of trucks wait to cross the Channel. In response, the British authorities have promised to equip highways in the area with portable toilets.
The European Union is Britain’s largest trading partner, representing 43 percent of its exports last year and 52 percent of imports, according to official figures. Mr. Johnson’s government has pledged to spend over £700 million, about $935 million, on infrastructure, jobs and technology at the border, and British and French officials have vowed that crossing points will be ready in time for the post-Brexit era.
Yet tens of thousands of British businesses were not ready for Brexit as of October, government officials told lawmakers last month, with more than a third of them expecting an extension to the transition period.
“The biggest potential cause of disruption are traders not being ready for controls implemented by E.U. member states on 1 January 2021,” Michael Gove, the minister in charge of Brexit negotiations, wrote in a letter to transportation companies that the British news media reported on in September.
The letter caused anger among drivers’ union representatives and industry leaders, who said they had received scant details on the preparations needed to comply with the new rules.
Richard Burnett, the chief executive of the Road Haulage Association, said last month that it was vital for transportation and goods companies to be prepared whether there was a deal or not.
“The message is clear,” he said: “If the paperwork’s not right, the goods won’t cross.”