It looks like there has been a heist in the Calais Wine Superstore, with some floor displays stripped of the box towers of prosecco, pinot grigio and cabernet sauvignon that are usually piled up to the ceiling.

The wine warehouse has become a victim of its own success. Its “beat Brexit bonanza sale”, which included three-litre boxes of the South African red Namaqua being marked down to £3.99, has whipped its British clientele into a shopping frenzy amid fears Brexit would call time on their regular cross-Channel booze runs.

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“Last weekend was manic, we’d never seen anything like it,” said Marco Attard, the co-owner of the Calais Wine Superstore. “We were so taken by surprise in February and early March that we sold out of many lines and have pressed the panic button to get more stocks in.

“People are realising they might not be able to come to Calais and bring back the same amount of wine if we get a hard Brexit. They are stockpiling.”

The soulless industrial estate that fans out from the port and the wine store, with its billboards advertising new world winemakers such as Jacobs Creek and Concha y Toro, is a far cry from the holiday brochure image of the French wine industry with its grand chateaux and artisan vineyards.

But the Calais wine trade, a little Britain where the signs are in English and the prices in sterling, is less about aesthetics than pricing.

Facebook Twitter Pinterest Keith Willcocks, Brian Willcocks and Ray Lynch with their wine loot. Photograph: Martin Godwin/The Guardian

Direct Ferries says sales of day trips from the UK to France are up 25% over the last five weeks on a year ago. “With UK supermarket retailers warning about food shortages and anxieties about what might happen in the event of no deal, it is no surprise that we are starting to see more day trips across the Channel,” said Direct Ferries’ chief executive, Sean Cornwell. “Our expectation is that demand will continue to increase in the weeks ahead.”

On the opposite side of the road to the Calais Wine Superstore, billboards advertising “wines under £2” belong to Majestic Wine; business is booming there too, with UK orders up 40% over the last four weeks.

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In its latest annual report, Majestic Wine admitted Brexit may effect the viability of its two French outposts, which are reliant on British shoppers buying in bulk to take back home.

On Thursday, David Nicholls and Steve Chappelle were queuing in Majestic Wine to pay for 300 bottles of wine for a wedding in May. “We wouldn’t have come this early without Brexit,” said Nicholls, who thought the round trip from Canterbury, in a transit van, had saved them £1,000.

The Wine and Spirit Trade Association (WSTA) has already warned UK wine prices could go up after Brexit. Added to this, last month the duty on still wine increased by 7p to £2.23 and by 9p to £2.86 on sparkling.

If the UK leaves the EU without a deal, shoppers will also face new trade tariffs. The average price of a bottle of still wine in the UK is £5.73 but the combination of these extra costs could add an extra 20p to the price, the WSTA said.

Facebook Twitter Pinterest UK supermarkets say the price of other imported food, such as French cheese, could also go up after Brexit. Photograph: Sophia Evans/The Observer

In the Calais Wine Superstore tasting area, Nigel and Marilyn Kay, from Dorset, were sipping wines to find alternatives as their favoured brands were sold out. Nigel, a remainer, said he was “outraged” by Brexit and not just because it would dampen his wine-buying ability. “Brexit is restricting my freedom of movement – my ability to travel,” he said.

“You can save a lot of money,” Marilyn said, referring to their wine-buying trip. “There are some really drinkable everyday wines. We come here for what we call slurping wine.” They also pop to Aldi in Calais to buy rosé, she added.

Outside in the windswept carpark, Keith Willcocks, his cousin Brian Willcocks and Ray Lynch, three retirees from Surrey, were piling boxes of wine into the boot of a Honda. “It’s enough for a couple of glasses of an evening,” said Keith Willcocks, of their fairly regular wine-buying outings in Calais. “It’s surprising how quickly it goes. We knew we’d have to go before it [Brexit] happened.”

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Lynch added: “We’ve got the time and the money, so why not?”

The men, who both voted to leave the EU, accepted their wine runs could soon be history but were happy to suffer the consequences of their vote. “I don’t regret it, I made the right choice,” said Lynch, adding that the government had “cocked the whole thing up”.

In the event of a hard Brexit, the WSTA has said it was likely the duty-free rules that apply to the rest of the world would apply to alcoholic drinks from the EU as well, meaning a four-litre wine allowance. “This means that it would no longer be possible to buy wine and spirits for personal consumption, duty paid in the EU, and bring them back into the UK unrestricted,” said Simon Stannard, the WSTA’s director of European and international affairs.

Meanwhile, the Calais wine trade is riding high on a pre-Brexit boom but business owners are keenly aware the party could soon be over. “A hard Brexit means the booze cruise is dead,” said Attard.