Britain must increase home-grown food production and build stronger supply chains to face Brexit uncertainties, the National Farmers Union has said.

In an annual calculation to draw attention to the UK’s decline in food self-sufficiency, the NFU said the national larder would be bare this Sunday if Britain opted for a cliff-edge departure from Europe and imports became unavailable.

Such is the reliance on imported food that Britain does not even produce enough of staples including potatoes, beef, milk and cheese to feed the nation for an entire year, it found.

“Food self-sufficiency statistics have always been an important measure of the nation’s ability to feed itself. But since the UK voted to leave the EU, and with trade negotiations starting, the supply of British food is now seen in a very different light. Government recognition of farming’s enormous contribution to this country will be vital in the coming weeks and months,” said Meurig Raymond, NFU president.

The NFU has called on the government to grasp Brexit as an opportunity to reverse a steady decline in self-sufficiency over the past 30 years.

The farming union said that with the right support it would be easy to replace the salad mountain imported from east Africa, pork from countries such as Denmark and New Zealand, and beef and dairy from other EU countries such as Ireland.

“We’re not advocating a fully self-sufficient nation – we recognise the need for importing food which can only be produced in different climates. But what we should be doing is maximising on the food production we are good at,” said Raymond.

The NFU said 30 years ago the country produced 80% of its own food, but now it was 60%.

“If we carry on at that rate of decline we could easily be down to 50% in 10 years, and that is a pretty insecure place to be,” said Raymond.

“The two main responsibilities of any government are to defend its people and feed its people. We are already among the least self-sufficient countries in Europe and dropping to below 50% would be a very insecure position to be in,” he added.

With unseasonally cold snaps in places like Spain causing vegetable shortages this year and the threat of drought in big exporting nations like the US, over-reliance on other countries for food is a national security risk.

Raymond said he believed the public appetite was there for British food and “not ready for hormone-treated beef”, which is banned by the EU but could find its way on to UK supermarket shelves after Brexit under a trade deal with the US.

The NFU said farming was such an important contributor to the economy – generating 3.9m jobs and revenues of £109bn a year – that the government needed to recognise the sector’s needs.

The NFU said it wanted a clear transition period after Brexit, during which the UK will remain in the customs union, that the UK remained within the external tariff area, which would guard against the country being flooded by cheap imports of questionable quality.

Raymond said the environment secretary, Michael Gove, was beginning to recognise the importance of a transition period in behind-the-scenes talks with farmers, although “there is no commitment”.

The NFU plea came as 14 farming organisations, including the British Poultry Council, the Soil Association and the Country Land and Business Association, set out a list of Brexit demands.

They too want the government to manage the risk of Brexit and stay within the customs union during a transition period. They also want “a fully functioning immigration system” that recognises the reliance food producers have on EU workforces beyond the harvest season.

Raymond said the NFU also wanted the government to stop viewing these workers as unskilled but rather as highly competent workers on whom the food growing and packing industry relied.