More than half of the UK’s food will come from overseas within a generation, as a rising population and stalling farm productivity combine to erode what remains of the UK’s self-sufficiency, according to farming leaders.

The UK’s failure to produce more food will leave households more vulnerable to volatile prices and potential shortages, the National Farmers’ Union will say at its annual conference on Tuesday. The farming body will call on politicians to encourage new investment in farming, and develop a national plan for a higher degree of food self-sufficiency.

Meurig Raymond, president of the NFU, warned: “The stark choice for the next government is whether to trust the nation’s food security to volatile world markets or to back British farming and reverse the worrying trend in food production. I want to see a robust plan for increasing the productive potential of farming, stimulating investment and ensuring that the drive to increase British food production is at the heart of every government department.”

The NFU cited “poorly crafted regulation”, including EU and UK policies that have “over-emphasised environmental rather than production outcomes and complicated the busienss of farming”, and farmers having weak bargaining power with big retailers as key problems affecting agriculture.

Farmers meeting in Birmingham are expected to demand more attention from politicians ahead of the general election, when rural votes could play an important role in deciding the make-up of the next government. The Conservatives dominate in rural areas, but many key Liberal Democrat constituencies have a farming base.

Farming production is worth about £26bn a year, while the broader food industry accounts for about £103bn to the UK economy, more than the car and aerospace industries combined, and represents about 3.5m jobs.

According to projections by the NFU, on current trends the UK will reach a tipping point in about 25 years, beyond which a majority of our food will have to be imported, unless governments take strong action to improve food production and protect consumers from a future of relying on food bought from abroad.

Self-sufficiency in food in the UK has been eroded since the 1980s: about 60% of food currently consumed here is grown here, down from nearly 80% in the mid 1980s, even though more varieties of food previously thought exotic are now grown in the UK.

The problems created for British farmers when cheap imports flood the UK’s market have been illustrated in recent weeks. A glut of dairy products on international markets has sent prices to farmers plummeting, driving thousands out of business and threatening a future in which the UK has to import its fresh milk.

The Department for Environment, Food and Rural Affairs said the government was creating opportunities for farmers. “We are helping the industry become more competitive, at home and abroad, by opening up record numbers of international food markets to export our produce, making it easier for our schools and hospitals to buy local, helping consumers to choose UK products through improved country-of-origin labelling, and investing in cutting-edge technology such as GPS-guided tractors.”