Alcohol deals in the big four are now so competitive that even the retailers' Calais drink warehouses can't undercut them.
A 24-pack of John Smith Extra Smooth costs £18.05 at Tesco Calais and £18.99 at Sainsbury's Calais, but both retailers sell it for £10 per 15-pack in the UK considerably less per can.
Tesco's UK stores also sell 24-packs of Foster's cheaper than Tesco Calais, while Sainsbury's UK stores undercut their Calais counterpart on Smirnoff Blue vodka (1 litre) by a hefty £3.64 per bottle.
Carlsberg, Bombay Sapphire and Corona are also cheaper in the UK than Calais, and other drinks are only marginally more expensive. Sainsbury's Calais sells Carlsberg for just 3p more per 24-pack in the UK, for instance.
"We have great value promotions on a number of lines in UK stores so there will be occasions when UK store prices are cheaper than those across the Channel," admitted a Sainsbury's spokeswoman.
The unfavourable exchange rate has exacerbated the situation. However, wine and Champagne were still cheaper in Calais, said the Sainsbury's spokeswoman, speculating that the booze cruise was being "gentrified" as middle-class shoppers continued to head to Calais for wine bargains, while lager and spirits drinkers stayed away.
Tesco insisted that prices remained £1.50 on average more expensive in the UK. The Calais depot "offered a larger range than any UK store with very knowledgeable staff and the opportunity to taste the wines before buying", added a spokesman.
Neither Sainsbury's nor Tesco would reveal sales or footfall figures for their Calais depots. But in its June trading update, Majestic said its Calais warehouse had seen a major drop in customer numbers. The number of transactions fell 27% and typical spend dropped 7%.