On 24 June last year, the few hundred residents of a temporary village, hidden from view in the middle of a West Sussex soft fruit farm, received letters. They were signed by David Kay, the managing director of the Hall Hunter Partnership, a business that grows 10% of the UK’s strawberries, 19% of its raspberries and a whacking 42% of its blueberries across thousands of acres, of both glasshouses and polytunnels. The recipients were his seasonal workforce, some of the 3,000 pickers from Bulgaria, Romania and elsewhere who come here each year to get the harvest in, and without whom the business would simply not exist.

“I wanted them to know that in the face of the vote for Brexit we would hang together as a family,” he says now, standing amid the mobile homes his workers live in during the summer months. The dwellings come dressed with satellite dishes pointed at news channels in Bulgaria, and pylons delivering high-speed wifi. Some have planted gardens. Tesco Direct delivers their groceries; coaches take them out on excursions.“I’m responsible for both a fruit farm and 2,100 beds,” Kay says. “That morning I met a lot of very sad and confused workers. For me, personally, it was a shock.”

Kay may have wanted to reassure his employees in the immediate aftermath of the vote, but 11 months on their status is no clearer. Indeed, this tidy little village could now stand as a blunt symbol for one of the most serious but little talked about issues arising from the Brexit negotiations: the continued ability of this country to feed itself, if the deal goes wrong. Opponents of EU membership talked during the referendum campaign about sovereignty and control. They railed against the free movement of labour. What they didn’t mention is the way the British food supply chain has, over the past 30 years, become increasingly reliant on workers from elsewhere, both permanent residents and seasonal labour.

Last month, as parliament wrapped up for the general election, the Environment, Food and Rural Affairs Select Committee quietly published a short paper called Feeding the Nation: Labour Constraints. As it reported, around 20% of all employees in British agriculture come from abroad, these days mostly Romania and Bulgaria, while 63% of all staff employed by members of the British Meat Processors Association are not from the UK. Around 400,000 people work in food manufacturing here, and more than 30% of those are also from somewhere else. If free movement of labour stops, the British food industry won’t just face difficulties. Some parts will shudder to a halt. Shelves will be emptied. Prices will shoot up. And right now, none of those charged with negotiating Britain’s exit from the EU are making promises that this scenario will be averted. Many of them aren’t even engaging with the issue.

Facebook Twitter Pinterest Strawberry farm workers picking. Photograph: Alex Lake/The Observer

The soft-fruit business of which Hall Hunter is a part is right at the sharp end of that. “From February to November we need 29,000 seasonal workers across the sector,” says Laurence Olins, chair of British Summer Fruits, the crop association for berries, which account for one in every £5 spent on fruit in the UK. “And 95% of those are non-UK EU citizens.” The industry has tried to get UK nationals to do the work but they’re simply not interested. “Our hope is for some sort of permit scheme,” Olins says. “But if, say, we get only half the permits we need, we will simply have only half the size of the industry.” The 29,000 non-UK workers they have are therefore vital. And, what’s more, the number required is growing.

Kay agrees. “We’ve worked with job centres and with ex-prisoners, but British people don’t want to do these jobs.” Instead, he says, he gets a steady supply of highly educated and motivated eastern Europeans, most of whom have some connection to farming because their families still have smallholdings. “We have a return rate of 76% each year,” he says, “which means we retain a skills base – 70% of our management arrived here as pickers and worked their way up the ranks.” He shows me a list of the 20 most important people in the company and it’s littered with Slavic surnames – 20 nationalities are represented on site.

Some have settled here, put their kids into schools and taken UK citizenship. But many more are just seasonal, coming and going at short notice. Every single one is interviewed for a job by a member of Kay’s team; they run temporary recruitment centres in town halls and civic libraries across eastern Europe.

Facebook Twitter Pinterest At Heck Sausages in Yorkshire, 85% of the workers in production come from eastern Europe. Photograph: Alex Telfer/The Observer

At the farm, amid glasshouses of glossy strawberries planted at shoulder height for easy picking, I meet Zyulfie Yusein, 29, and Nikoloy Kolev, 34. Both are from Bulgaria. Both are graduates. Both first came here to earn a little money as students, returning home with their earnings. Over the years, they’ve stayed longer and longer. “It’s a great job,” says Yusein. Kolev agrees. “We work as a team and the team is like family.” But both say the Brexit vote has changed everything. “I worry about the future,” Yusein says. “My friends worry too. The vote made me feel unsafe.” Kolev says, “Going back is not an option but what am I going to do?” They are warm, bright, friendly people, but the tension just beneath the surface is palpable. They are already experiencing the downside. The Brexit vote has weakened the pound by up to 20%. Their salaries are worth far less at home than once they were.

And the message is getting back to their friends. “Some of the seasonal labour is choosing not to come to the UK because of the value of sterling,” Olins says. “If you can go to work in a Euro country like Spain, rather than Britain, it’s worth doing so.”There used to be 10 applicants for every picking job in the UK. Now there are three. “The candidates we’re getting are older, they have fewer skills, their English is worse.” Is that just down to Brexit? “The media in the home countries has been reporting attacks on immigrants to the UK,” he says. The mood here has changed. And it risks imperilling the harvest British citizens don’t want to help bring in.

On 26 July, 2016, a little over a month after the referendum vote, representatives of more than 40 food and drink associations gathered in the meeting room on the sixth floor of the Food and Drink Federation’s HQ on London’s Bloomsbury Way. Here were representatives of the British Poultry Council and the Federation of Bakers, the British Growers Association, the National Association of Cider Makers and many more besides. They were joined by civil servants from Defra, the Food Standards Agency, the Department for Business, David Davis’s Brexit department and HMRC. The meeting had been called by Ian Wright, a former executive at drinks company Diageo who now heads the Food and Drink Federation. The meeting was to coordinate a response to Brexit. And top of the agenda was the issue of labour. The same group has met every month since.

We’ve worked with job centres and with ex-prisoners, but British people don’t want to do these jobs

“It’s fair to say that we started out with a degree of surprise at all levels,” Wright says. “Very few ministers or civil servants understood the nature of the food-chain workforce.” He believes they have managed to get the message across, but that’s a very different thing to dealing with the issue, given the refusal by Downing Street to be drawn on their negotiating positions. “Right now, there’s a great deal of work going on to define the choices the prime minister will have to take to sustain the variety and complexity of the food supply chain.” The alternative, he says, is fewer choices for consumers or sources of labour from outside the EU.

Early in this election campaign Labour’s Brexit spokesman, Keir Starmer, made a commitment to guarantee the rights of the estimated 3.9 million EU citizens living in the UK on day one of a Labour government. David Davis met this with a soothing assertion of a swift deal to secure those rights. That’s not surprising. The non-UK citizens here are mostly of working age and economically active. The 900,000 UK citizens in Europe are mostly pensioners living out their retirement on the sun belt in increasingly poor health. Theresa May’s government is desperate not to have them sent back for fear of the pressure they will place on the NHS.

But what matters is not those living here full-time but the seasonal workforce that comes and goes. Until 2013, there was a seasonal-labour permit scheme which, ironically, was abolished, because the EU free movement system was deemed to be working so well. A replacement would be needed. Pushed for a number of permits required, Wright suggests “around half a million”. Hard-line Brexiters, committed to an end to the free movement of labour, might well find this unpalatable. Indeed, one of the big food-sector bodies told me they received off-the-record calls from civil servants warning them to shut up, because they had been quoted in newspapers talking about the seriousness of the labour supply to the food chain. “We were told we would just enrage the hard-line Brexiteers,” a member of the body told me.

The problem is compounded because some sectors need a huge mass of workers. Others need very few. In some areas of the food chain, it can be down to just a few dozen people who keep the whole thing running. For example, under Food Standards Agency rules, an abattoir in England, Wales or Northern Ireland cannot operate unless the animals on the way to slaughter are overseen by one of their vets. This is work British vets don’t want to do. They would rather be out on the farm with livestock in the prime of their lives, or dealing with domestic pets. As a result, at least 85% of vets in British abattoirs are not from the UK. Apparently, the majority are Spanish. And if they couldn’t get into the country to do the job, the meat supply chain would collapse.

While Ian Wright is good at the diplomatic phrase, others feel less constrained. In the months running up to the Brexit referendum, Tim Lang, professor of food policy at London’s City University, co-authored a briefing paper on Britain’s dependency on EU member states for its food. It dealt in detail with seasonal labour from the EU. He can be forgiven for wondering why he bothered. “The civil service is dispirited and uncertain of what they’re doing because they haven’t been given any signals,” Lang says now. “There’s not a bleep about food policy coming from ministers. There has been a stunning silence from Andrea Leadsom, the Defra minister, on this matter of national importance. Basically, if on March 31, 2019, migrant labour is not sorted the food system is fucked.” And then he says, “I hope those who voted Brexit and who still want to eat British are prepared to go to Lincolnshire in winter to pick vegetables.” Or as Wright puts it, “Food is at the heart of national security. If you can’t feed a country you haven’t got a country.”

Facebook Twitter Pinterest Hall Hunter’s farms rely on EU workers. Photograph: Alex Lake/The Observer

For five years as a food reporter for the BBC’s One Show, I used to travel the country from one strip-lit food production unit to another, looking at exactly where our food came from. The ethnic mix was always striking. The media were forever talking about a British food revolution; of a homegrown improvement in quality at both small and large scale. And the companies were indeed British, but so many of the people doing the actual producing were not. I visited cake factories where the health and safety notices were in both English and Polish; was given tours of vegetable processing plants where the floor managers needed a smattering of four eastern European languages to get by.

I take a train north from Hall Hunter’s fruit farm, to the North Yorkshire home of Heck Sausages, run by Debbie and Andrew Keeble. In just four years their innovative range of gluten-free sausages – from pork and apple, through square to non-meat alternatives – has been stocked by all the major supermarkets. Their turnover is projected to reach £18m this year and they are about to move into a new plant which will enable them to run multiple production lines.

Facebook Twitter Pinterest Debbie Keeble of Heck. Photograph: Alex Telfer/The Observer

The only issue is workforce, which will have to double. Of the 60 people currently working in production, 85% are from eastern Europe; like Hall Hunter, Heck can’t get British people to do the work. I ask Debbie Keeble what an end to free movement of labour would mean to her business. “It would be cataclysmic,” she says. “No one here will take these jobs.” The Heck factory is in an area that voted strongly for Brexit. “During the referendum, campaigners were going on about people coming over here taking our jobs. Well, they’re not, because nobody here applies for them.”

Mostly she says it’s word of mouth, with new employees coming either directly from Latvia or Romania or from within the communities in the UK. I talk to one young Romanian woman, Georgeta Iclodean, who talks about getting increasing amounts of hassle from Border Agency officials when she re-enters Britain. “I make sure to have all my papers with me now,” she says.

I meet 34-year-old Vladim Protasovs from Latvia who came to Heck in 2014 and has risen to be one of the line managers. “I like working in the UK,” he says. “It’s a very big difference from Latvia.” But Brexit has changed everything. “My children are settled in school here,” he says. “If we had to go back it would be so hard, not just for me but for them. There’s lot of people who want to come from Latvia to work in the UK, but they are worried. I call my friends to say there are jobs but they don’t want to come.”

Then he says: “What happens next?” It’s a good question. The truth is nobody knows, not the business leaders, not the diplomats and certainly not the politicians. The prime minister and her team have portrayed negotiations as a game of poker, used the language of hands unrevealed and bluffs, while failing to recognise that the analogy doesn’t work; poker is a winner-takes-all game and Britain cannot afford to lose everything.

The Brexit deal isn’t just about vague concepts of nationhood. It isn’t simply about international standing or the ebb and flow of trade.It’s about the lives of individual people like Protasovs and Iclodean, Yusein and Kolev; the ones prepared to do the back-breaking jobs British people are not. What’s more, this is not just their crisis, to be worked out in anguished letters home. It’s ours too. Because without them and the half a million seasonal workers like them, our very ability to feed ourselves, at a price we can all afford, is in peril. In the forthcoming Brexit negotiations that is what’s really at stake.