The Eurotunnel in Coquelles, near Calais | Philippe Huguen/AFP via Getty Images EU-UK trade lifeline faces Brexit disruption, diplomats told Representatives of the EU27 will visit the Channel Tunnel’s French entrance Tuesday.

Brexit risks disrupting the smooth and rapid operation of a key trade lifeline between the EU and the U.K., diplomats will be told Tuesday while on a VIP tour of the French side of the Channel Tunnel.

A group of EU27 Brexit experts will be shown around the site at the request of GetLink, the company that runs Eurotunnel, on a "study visit."

The tour was listed in a leaked calendar of Brexit-related meetings and activities by diplomats and technical experts in the Council of the EU, obtained by POLITICO earlier this month.

The Council has refused to say which diplomats are attending or anything about the itinerary, but the trip will allow senior GetLink staff to brief the group on the importance of the undersea rail line to EU-U.K. trade — part of a lobbying campaign by the company to raise awareness over the consequences of a no-deal exit.

Diplomats will be told about the near 2 million trucks that shuttle through each year, and the millions of parcels a month that make it across as part of an ever-increasing e-commerce market. All alternatives, for instance through other ports, would take up to four times longer for hauliers and would struggle to offer the same capacity.

Components worth €4.3 billion make the journey through the tunnel annually, part of the just-in-time supply chains carmakers like BMW rely on.

And any extra friction brought about by Brexit in the form of custom checks or a failure to set up ample facilities for controls on the French side will cut into the Channel Tunnel's biggest advantage: speed.

“It’s like a Swiss clock, always running,” Anne-Laure Descleves, the tunnel’s director of communications, said of traffic at the site at present. “Day and night, the fluidity is spectacular to see.”

The "Chunnel" opened in 1994 and carries 27 percent of French exports to the U.K. by value and some 42 percent of French imports back from the U.K., according to a study by Ernst & Young commissioned by GetLink, the recently rebranded operator of the Eurotunnel services, published last month. The link is also the conduit for over 20 million passengers a year via Eurostar and shuttle services, along with 2.6 million cars and over 53,000 coaches.

A no-deal Brexit throwing up new customs checks and requirements for sanitary controls on agri-food products could throttle the only fixed link between the U.K. and continental Europe, just as the U.K.'s major EU trading port at Dover warns of its own capacity restrictions.

The English port expects major problems if customs checks are needed because it has no room to expand. The tunnel has the advantage of more space around its entrances on either side, but Brexit still poses potential problems. The rapid 35-minute transit time between the highway around Folkestone and landfall in Calais make it a key stretch of the land-bridge that links Ireland's food producers with the EU27.

“Competitiveness on this route is based on a high frequency of highly efficient and fast crossings, ideal for supply chains relying on just-in-time production and delivery processes,” said Pauline Bastidon, the Freight Transport Association’s head of European policy, who has also visited the tunnel entrance on the French side.

Even though more trucks rumble through the maritime route at the Port of Dover each year, they typically carry less valuable goods than those that use the nearby tunnel route. Trade through Dover came in at just £119 billion in 2016, almost £20 billion less than that which runs through the Channel Tunnel.

Dutch flowers and food produce make up just some of the trade heading under the Channel, but the link is also key to Germany's automotive industry. Components worth €4.3 billion make the journey through the tunnel annually, part of the just-in-time supply chains carmakers like BMW rely on.

To maintain the pace via the tunnel, Bastidon said deals will need to be worked out on any new sanitary and phytosanitary checks on food, for example, to make sure that controls don't slow down the flow of goods at the border, located in France under the terms of the bilateral treaty between the two countries. More extensive border posts will almost certainly be needed on the French side but it is not clear whether they could be constructed in time. A Swiss-style waiver on security and safety standards will also be needed and both sides will need to agree to firm up mutual recognition of driver qualifications, Bastidon said.

But all of that would require detailed agreements between London and Brussels as part of the Brexit talks. With the overall question of what customs arrangement will exist post Brexit still unresolved, the negotiators are yet to touch on such details.

For its part, GetLink argues that it has a track record of adaptability. It already has had to grapple with bringing in exit checks for all passengers leaving the U.K. in 2015, proving that it can switch operating procedures when needed, said Descleves.

Simon Marks contributed reporting.