Some of the U.K.'s biggest supermarkets have joined the National Farmer's Union in urging the government to seek access to the single market after it leaves the European Union.

Marks and Spencer (MAKSY) , Sainbury's (JSAIY) and Morrisons (MRWSY) are among the 75 signatories from the food producing industry, which represents more than £92 billion ($115.8 billion) in turnover and employs almost a million people, calling for continued access to labor and "best possible single market access after Brexit."

Food producers and farmers fear that a Brexit deal that does not allow foreign workers could significantly raise costs.The group warned that "food security, food safety and hygiene, stewardship of the countryside and affordable food is at risk" if single market access is taken away.

"For our sector maintaining tariff-free access to the EU single market is a vital priority," the letter said. "It is where 75% of our food exports go, so all our farming and food businesses wish to achieve this outcome."

"The sector needs access to EU and non-EU seasonal and permanent labor, alongside assurances that EU workers already working permanently in the U.K. are allowed to remain," the letter added. "This access to labor is essential as it underpins the UK food chain's timely delivery of high quality affordable food to consumers."

Tariff-free trade has been a sticking point between the U.K. and the EU even though formal negotiations have yet to start. Prime Minister Theresa May said she will trigger Article 50, the official exit clause, by the end of March 2017, kicking off the formal negotiations process.

This week British lawmakers were left embarrassed after an aide's notes were photographed as she left Westminster. The notes included "what's the model? Have your cake and eat it" a light-hearted sentence that has been used to illustrate single market access.

European leaders so far have not been open to the idea of Britain leaving the bloc but keeping tariff-free trade.

By 10:45 GMT in London, Sainbury's stock was down 1.13%, Morrisons lost 1.01% and M&S slipped 1.55% amid a wider market sell off. The overall FTSE 350 Food Producers Index has fallen 6.14% since the June 23 Brexit vote.

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