Farmers have called on the Government to make food security a top priority on the day the country's cupboards would run bare if households relied only on British produce.
A long-term decline in self-sufficiency that has stagnated at around 60% in recent years means around three quarters of the shortfall is imported from the rest of the European Union.
Farmers have also been "wrangling" with the impact of the summer heatwave, which has pushed concerns around food production into "sharp focus", the National Farmers' Union (NFU) warned.
President Minette Batters said: "This has been a real test for Government to show the farmers and the many concerned members of the public that they think that our ability to produce food in this country is truly important.
"We strongly believe that every British citizen should be entitled to a safe, traceable and high-quality supply of British food that is produced to some of the highest animal welfare and environmental standards in the world.
"Home-grown food production must have the unwavering support of Government if we are to achieve this post-Brexit."
Prime Minister Theresa May insisted last month that the public should take "reassurance and comfort" from Government preparations for a no-deal Brexit after ministers suggested food and medicines would be stockpiled in case of shortages.
The NFU highlighted figures from the Department for Environment Food and Rural Affairs that show self-sufficiency levels have hovered at around 60% for most of the last two decades, compared with up to 75% in the late 1980s and early 1990s.
"The statistics show a concerning long-term decline in the UK's self-sufficiency in food and there is a lot of potential for this to be reversed," Ms Batters said.
"And while we recognise the need for importing food which can only be produced in different climates, if we maximise on the food that we can produce well in the UK then that will deliver a whole host of economic, social and environmental benefits to the country.
"The UK farming sector has the potential to be one of the most impacted sectors from a bad Brexit - a free and frictionless free trade deal with the EU and access to a reliable and competent workforce for farm businesses is critical to the future of the sector.
"And as we replace the EU's common agricultural policy, we must keep a sharp focus on what productive, progressive and profitable farm businesses need from a domestic agricultural policy."
A Defra spokeswoman said: "The UK produces the majority of its own food.
"However, it is necessary to import some foods as they cannot be grown in the UK, and to ensure an excellent level of food security."