Those who want to keep close ties to the bloc point to warnings that quitting a customs union could cause chaos at ports and cost firms more than $25 billion a year.
Certainly, Mrs. May could pay a high political price if she gets things wrong.
Here at Dover, within eight minutes, trucks can normally disembark, move out of the port and onto the highway — where most pass a Brexit-inspired mural by Banksy, depicting a worker chipping away at a star on the European Union flag.
Only around 2 percent of trucks carry goods from outside the European Union. Those require a customs clearance, but that is done at separate centers away from the port and can take anywhere from 20 minutes to several days, said Richard Christian, head of policy and communications at the Port of Dover.
In the absence of the customs union, every single truck would, in theory, be subject to such a close examination. Yet, even a modest, two-minute delay in processing truck arrivals could cause a 17-mile line of traffic, says Mr. Christian. He recalled disruption along those lines following strikes that brought gridlock, leaving trucks waiting for hours on highways outside the port and on the other side of the English Channel in France.
“There were supermarket shelves empty and cars not being built; we know what happens when traffic can’t move across the channel,” he said.
Officially, Mrs. May is committed to quitting Europe’s customs union because continued membership would prevent Britain from striking independent trade deals — a primary selling point of the Brexiteers.