The $20 trillion European economy is built on open borders for delivering fresh English lamb to butchers in Milan or German disc brakes to BMW in Oxford — not in days, but hours.

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British negotiators remain resolute that a new free-trade accord can be hammered out with Europe by October, or if not then, by November, or ­December, or January — or before the stock and currency markets begin to freak out.

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Yet, at the same time, the British government is also warning British consumers and companies that they should brace themselves for a cold-turkey withdrawal, a “no-deal Brexit” or “Brexit doomsday” — causing some degree of panic.

“I believe we will get a good deal, we will bring that back from the E.U. negotiations and put that to Parliament,” Prime Minister Theresa May said in an interview with the BBC that aired Monday. But she added that if Parliament rejects her proposals, then the alternative “would be not having a deal.”

May’s government once sang a song of “Global Britain,” a vision for a 21st-century trading dynamo, exploiting new markets in new lands. But now Brexit Secretary Dominic Raab is encouraging drug companies to stockpile extra medicine in case supplies cannot get onto the island after a no-deal Brexit.

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Cadbury has started hoarding chocolate for fear that ingredients will become difficult to obtain.

On Thursday, the French minister for European affairs warned that British trains and planes might not be allowed into France without a deal, while the governor of the Bank of England told the cabinet that a no-deal Brexit would wreak havoc rivaling the financial crisis of 2008.

In its latest assessment, the International Monetary Fund warned Monday that the British economy would incur “substantial costs” without a deal in place.

It has gotten so bad that British authorities have had to downplay rumors that the army would be deployed to maintain civil order.

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The prospect of a Brexit doomsday has generated a summer of scary newspaper headlines, think-tank analyses and government reports that envision British automobile and airplane manufacturing stalled, grocery store shelves emptied and massive traffic jams of trucks idling for miles, waiting to get through the Eurotunnel and British ports.

If London “crashes out” of Europe’s enormous single market and regulatory controls, Britain could find itself suddenly branded a “third country” under E.U. trade rules, subject to not only quotas and tariffs but also inspections at border-control stations.

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It’s the inspections that truly terrify many.

In the blink of an eye — on March 30, 2019 — a crate of fresh Welsh oysters may no longer be whisked through the fast lanes at the Eurotunnel from Folkestone in south England to Calais on the French coast, and then quickly on their way to a bistro table in Paris.

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Non, non, non. Instead, the hapless shellfish could find themselves shipped to Dunkirk in France and subject to “sanitary inspections,” a regime that often requires a government veterinarian to open the load, take a sniff, pull a sample and hand-carry it to a laboratory to check bacteria under a microscope.

Without a deal, British meats and seafood may be viewed as no different than a container-load of frozen chicken from Malaysia, which can take 72 hours to pass through an E.U. port.

Ralf Speth, the top executive of Britain’s largest car manufacturer, Jaguar Land Rover, warned last week that his factories might stop running and “tens of thousands” of jobs could be lost if the country fails to reach a deal with Brussels, according to the Telegraph newspaper.

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“Brexit is due to happen on March 29 next year,” Speth told an automotive conference. “Currently, I do not even know if any of our manufacturing facilities in the UK will be able to function on March 30.

“Bluntly,” he warned, “we will not be able to build cars if the motorway to and from Dover becomes a car park, where the vehicle carrying parts is stationary.”

Many Brexit fans call threats of a no-deal doomsday a bluff, an attempt to scare voters with exaggerated worst-case scenarios that won’t come to pass — a propaganda project to instill fear.

But polls show that a majority of voters believe a no-deal Brexit is more likely than not.

For more than four decades, Britain has been closely intertwined with the rules and regulations of the E.U., a trading bloc of 28 countries. Goods crisscross on a daily basis — insulin from Denmark, olives from Greece, computer boards from France — whizzing around Europe as if it were a single country.

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Today, a shipment of British strawberries travels through the Eurotunnel in 90 minutes, English motorway to French auto­route — a few minutes for the mostly Polish truck drivers to go through check-in, passport control and security at Folkestone, then 12 minutes to be loaded aboard the trains that shuttle the trucks through the tunnel at 90 mph in a trip that takes 35 minutes.

“There are no warehouses, no parking lots, no room for that,” said John Keefe, head of public affairs at Getlink, the new name for the Eurotunnel group. “The truck is the rolling warehouse. There’s no stopping. Everything is based on motion.”

In July, at a contentious cabinet meeting at the prime minister’s Chequers countryside retreat, May proposed a negotiating position that would keep Britain closely allied with European rules and regulations to save “frictionless trade.”

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Within hours, her Brexit secretary, David Davis, resigned; a day later, Foreign Secretary Boris Johnson followed him out the door, damning May’s proposal as “Brexit in name only.” Hard-line Brexiteers such as Johnson argue that it is better to leave the union with no deal than be vassals to Brussels.

Last week, Johnson compared May’s plan for Brexit to putting the country’s constitution in a “suicide vest” and handing the detonator to the E.U.

Big stumbling blocks remain between the E.U. and the United Kingdom, including how to avoid a hard border between Northern Ireland and the Republic of Ireland and along the Irish Sea.

If Britain were to exit with no deal in place, it would “quite likely to be the worst political, economic, and social crisis for a generation — maybe longer,” said Rob Ford, a professor of politics at the University of Manchester.

One of the problems, he said, is the false belief that no deal would allow the status quo to simply continue.

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“But it fundamentally doesn’t mean that in this situation,” Ford said. “No deal means you burn the whole house down, and I don’t think awareness of that is anywhere near high enough.”

Tim Lang, professor of food policy at the University of London and one of the authors of a recent briefing paper on Britain’s food security, said a no-deal Brexit would be “very risky indeed.”

Britain, like the rest of the developed world, has adopted the “Toyota management” philosophy that the best way to eliminate waste is to supply and procure only what is needed, exactly when it is needed, Lang said.

Businesses, on average, don’t hold more than three to five days of supply, he said. So if there’s no deal on March 29, five days later “shortages begin to happen if borders start existing again, if food inspections have to happen at borders, if paperwork takes longer.”

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It wouldn’t take long for the British public to feel the impact. Since the early 1980s, when Britain was about 80 percent self-sufficient in food, it has steadily slid to about 60 percent. The bulk of imported food comes from Europe.

“The wild ideologues who fantasize that a no-deal Brexit would be great? They are fat cats and rich usually and don’t realize that food has very, very fragile status,” Lang said.

Ian Dunt, author of “Brexit: What The Hell Happens Now?,” said that it was “quite hard to talk about the scale of the calamity, in food, in general trade, in the whole way society operates, without sounding hysterical.”

He said countries are, for good reason, cautious about the food they allow in. “They know if anything goes wrong, if someone gets sick and if a child dies, gets a disease from eating a burger, the political consequences of that will be severe.”

There are strict certification systems in place for food coming into the E.U. from third countries, he said. If Britain leaves without a deal and doesn’t match E.U. requirements, then food is “not going to get in.”

Jason Aldiss, managing director of Eville & Jones, a company that supplies official veterinarians (most of them from Europe) for slaughterhouses in England and Wales on behalf of the British government, said a true no-deal Brexit would be enormously disruptive.

Meat products would be slapped with higher tariffs, more inspections would be needed, and there would be fewer people to do them, he said.

It would be so disruptive that Aldiss doesn’t think it will happen.

“This is a doomsday scenario,” he said. “I do not believe that anyone is that stupid to permit us to crash out.”