Every day as many as 10,000 trucks rumble through the Port of Dover, whose towering white cliffs face continental Europe across the narrowest stretch of the English Channel.

Rigs loaded with French cheese, German car parts, and other European goods roll off ferries and onto British highways, while trucks hauling Scotch whisky and Welsh lamb bound for the Continent glide through passport checks in two minutes on average.

Traffic through the facility amounts to almost a fifth of all the U.K.’s trade in goods, worth about £122 billion ($165 billion) annually.

Brexit threatens to clog this crucial artery. Port officials warn that increasing the average time it takes trucks to clear customs by as little as two minutes could lead to 17-mile (27-kilometer) traffic jams.

And with talks in Brussels deadlocked and Britain’s withdrawal from the European Union fewer than 18 months away, the man who oversees the port says he doesn’t know whether to train more customs officers, look at land purchases to ease congestion, or just keep calm and carry on.

Once we understand what it is that has to be done, then we can come up with a better plan, says Chief Executive Officer Tim Waggott. At the moment, you tell me what I need to plan for.

It’s not only Dover. The U.K’s impending divorce from Europe threatens to disrupt ports across the country, from the dockyards of Felixstowe on England’s eastern flank to the Port of London itself, to say nothing of European shipping hubs such as Calais and Dunkirk.

Yet, as a so-called roll-on, roll-off facility operating on tight deadlines, with almost all its goods coming from or heading to the EU, Dover will be among the most affected by any Brexit complications.

The port is a tangle of roads that wind through parking bays and security checks to connect the M-20 motorway with the ferry terminals. The trucks rolling by sport license plates from all corners of the 28-nation trading bloc.

There’s a streamlined facility for EU goods and another section that performs customs checks on imports from outside the common market, a process that can consume anywhere from 5 minutes to 45 minutes per vehicle. Rejigging the operation to make all the goods go through non-EU processing would take months, if not years, of planning.

The head of the U.K.’s tax and customs body called Dover’s customs checks a major concern in a Parliamentary committee hearing last month.

Complicating matters further, there’s little room to expand to accommodate more slow-moving traffic. The port is hemmed in on one side by the sea and on the other by the sheer cliffs, Waggott says, pointing to a large map in the conference room of the Dover Harbour Board, which was founded in 1606 by King James I.

The UK has until early 2019 to negotiate a new trading relationship with the EU. If it can’t meet that deadline, it will revert to so-called third-country status. Waggott estimates that a no-deal scenario would require him to recruit an additional 250 customs officers.

The impact would ripple across businesses and Britain’s economy. The port is a key conduit for delivering goods to supermarkets and factories, which have honed their supply networks to precision. Long and unpredictable delays clearing customs would upend those schedules and prompt companies to seek alternatives.

It’s a location that deals with a truly vast amount of traffic, and a very small percentage of that gets checked at the moment in any way, shape, or form, says Andrew Meaney, a transportation analyst at Oxera, a consulting firm.

What you don’t want to happen, from a U.K. perspective, is to have industries starting to relocate to the Continental side of the channel because they can no longer rely on the time it takes to get from A to B. In a July report, Oxera estimated that any disruption at Dover resulting from a messy exit from the EU would cost businesses at least £1 billion a year, describing that figure as extremely conservative.

The port already operates near capacity, with a four-kilometer, zigzagging buffer zone to absorb overflow. If all the trucks that pass through Dover in a single day were lined up end to end, they would stretch from the port to London’s Stansted Airport, almost 100 miles away.

When serious backups have happened previously because of strikes by ferry workers in France or clashes with migrants attempting to enter the U.K. as stowaways, police have implemented an emergency procedure called Operation Stack, which allows trucks waiting to enter the port to spill onto the highway, clogging traffic for miles.

As if Waggott didn’t have enough to worry about, the U.K. government is scheduled to replace the information technology system used by the national customs service in March 2019, around the same time Britain is set to exit the EU, which could further hinder the transition.

On Oct. 9, the government outlined contingency plans in case no Brexit deal is reached. Acknowledging the chaos that such a scenario could create in ports like Dover, the government proposed shipments could be pre-notified to customs.

Like many business leaders, Port of Dover’s CEO favors a Brexit deal that maintains things as close as possible to the status quo and keeps traffic flowing. I’ve been asking for certainty since March, says Waggott. I’m still asking.