"It is looking exceptional this year," says William Pitts, as the torrent of golden wheat ears sweeps into the flashing blades below the cab of his combine harvester. In the next hour, about 50 tonnes of wheat will pour out of the combine into waiting trucks as it clears the field on Grange Farm in Northamptonshire. "We had a mild winter, rain when we wanted it and good sunshine too."

That is a relief for farmers after a rain-ruined 2012 and the frozen spring of 2013. But despite the prospect of a decent harvest this year, Britain's ability to feed itself is in long-term decline, with self-sufficiency falling from 78% to 60% in the last 30 years.

The National Farmers Union calculates that this Thursday would be the day that the year's food supply would run out without imports. "An alarming notion," says Meurig Raymond, president of the NFU. "But looking back seeing the downward slope in self-sufficiency says to me: this needs to change."

In July, MPs warned that the nation's ability to feed itself is threatened by "complacency" over the increasingly extreme weather driven by climate change and increasing competition for food as the world's population grows.

Pitts farms 1,500 acres of wheat and oil seed rape with his brother Andrew, who says consumers should be concerned: "People say if we don't grow it, we can buy it from abroad, but at what price? And whose wheat is it? There is a moral question: would that wheat have gone to Africa instead?" Andrew Pitts notes that in 2008, after a poor global harvest, the price of wheat doubled and that the unrest in Ukraine has affected wheat prices.

There are many reasons for falling self-sufficiency but the one most affecting Matthew Rawson, a major producer of brassicas like cabbage and broccoli, is the more eclectic tastes of today's shoppers. "My sprouts, cauliflowers and cabbages have been decline as people are eating other things," says Rawson, who is based in Yorkshire. "Did you see courgettes on the shelves in the 1980s?"

Supermarkets dominate the food market and are accused of replacing British produce with cheap imports. But, for fresh vegetables at least, Rawson says: "UK retailers will take British produce first and foremost if they can get it. When we need to dip into Europe, it's because the weather has been dreadful or it is outside the growing season."

However, Ali Capper, who grows Gala and Braeburn apples in Worcestershire, disagrees. She says English apples could be available for much more of the year, but get squeezed between the South American and European seasons by the corporations that determine the world supply. Despite that, English eating apples are one of the crops bucking the trend of longer term decline, if only a little. "Since 1990, self-sufficiency has improved, but it is still only 20% for eating apples," she says. "I am sure we can do better than that."

The cider boom has helped. "Cider is a quintessentially British drink," she says and about half her 100 acres of orchards grow the unique English bitter-sweet cider apple. But despite a huge surge in cider drinking, she says the growers are not benefitting financially: "It's great news for everyone, but the value is not coming down the supply chain."

Like apples, increased production of strawberries and asparagus are also displacing imports, while celery and broccoli are rising but not fast enough to keep up with demand. Other crops are falling rapidly, though. Broad and runner beans, tomatoes, and pears have tumbled by more than 20% in the past decade, pushed out by foreign competitors who can supply hungry supermarkets all year round. Peas, lettuce and leeks are also falling, though not as precipitously. The difficulty in finding people in the UK to harvest hand-picked crops like strawberries is another issue cited by farmers.

Many who work the land also blame the UK's higher and more costly food standards for falling self-sufficiency, by driving prices above those from abroad. Most controversial is the banning of pesticides because of their environmental harm, often contested. Failing to protect a crop from weeds, disease or insects can crash the yield says Guy Gagen, chief arable adviser to the NFU: "It's a disgraceful loss. It's as bad as tipping food away into a landfill."

But the biggest factor in Britain's ability to feed itself, says Gagen, is a failure to invest in new crops and smarter growing systems. "We have some of the best growing conditions in the world – plenty of water, good soil and sufficient light – and we are not taking advantage of them. We could produce more but we are not meeting our full potential."

He says the grain and butter mountains driven by EU subsidies in the past led governments to stop research into improved varieties and more productive ways of growing, which had traditionally been publicly funded. Yields of wheat and oil seed rape, for example, have flatlined since 1998, with the UK's rising population meaning more has to be imported. "We are not competitive internationally and people will not buy British just to be patriotic," Gagen says, naming the US, Australia, Ukraine, Russia, Argentina and Brazil as key competitors. "Brazil is just doing a better job. It's a bit embarrassing but they are showing us the way to go," he says, noting that nearby Paraguay is developing the ability to produce three crops a year. In January, former environment secretary Owen Paterson warned that the UK and Europe risked becoming the "museum of world farming" by not allowing genetically modified crops to be grown. The July report from MPs said the government's new £160m scheme for agricultural research was not enough.

Sarah Dawson, who is credited with bringing purple sprouting brocolli to the mass market in the UK from her Lincolnshire farm, agrees that research on improving yield and quality has fallen behind. "Funding and interest in science and technology has dwindled over past 15 years," she says. "If you look over to the European continent, other countries there are 15 years ahead of us. We don't have a plan to catch up driven by the government."

But Dawson praises a move to encourage the public sector like hospitals, schools and prisons to buy British, announced by prime minister David Cameron in July. "No one does more or works as hard in Britain today than our farmers," Cameron said. "By opening up these contracts, we can help them create more jobs, invest in their businesses and make sure people in our country have a healthier lifestyle." Dawson says: "It is quite serious, tangible and very welcome. But we could have done that years ago."

Despite the concerns on Grange farm, Andrew Pitts is admiring the quality of this year's wheat: "See the flintiness of that grain – the opaqueness – that means the grain has high-protein, which will make good bread." His farm is already hi-tech, with the laser-guided combine harvester and a satellite-monitored fertiliser regime, but he agrees more is needed. But he adds: "We are going to have two billion more people on the planet and we have to produce food with the same land and the same water. We have to learn to produce more with less. Science has kept us fed and watered for many past generations and that's what we need to do in the future."