The wettest autumn since records began, followed by the coldest spring in 50 years, has devastated British wheat, forcing food manufacturers to import nearly 2.5m tonnes of the crop.

"Normally we export around 2.5m tonnes of wheat but this year we expect to have to import 2.5m tonnes," said Charlotte Garbutt, a senior analyst at the industry-financed Agriculture and Horticulture Development Board. "The crop that came through the winter has struggled and is patchy and variable. The area of wheat grown this year has been much smaller."

Analysts expect a harvest of 11m-12m tonnes, one of the smallest in a generation, after many farmers grubbed up their failing, waterlogged crops and replanted fields with barley. According to a National Farmers Union poll of 76 cereal growers covering 16,000 hectares, nearly 30% less wheat than usual is being grown in Britain this year.

Britain is usually the EU's third biggest wheat grower but it will be a net importer for the first time in 11 years. "Our poll is a snapshot but it is extremely worrying. If this plays out nationally, we will be below average production for the second year in a row," said NFU crops chair Andrew Watts. "If the experts are to be believed and extreme weather is to become more frequent, we must look at ways of supporting the industry."

The diminished wheat harvest will add to growing concerns about the amount of food that British farmers can grow per hectare. According to a new analysis by the development board, UK wheat and oilseed rape yields have barely improved since the 1980s, despite genetic developments and better fertilisers. No one reason is given but severe and fluctuating weather is thought to have played a part.

Other crops have been badly damaged by the past year's severe weather. Oilseed rape and oats have suffered, and sugar beet, which is grown on 125,000 hectares mainly in eastern England, has been hit by a mystery condition that has stopped seeds germinating and has cut production by 50% in some areas. "The weather has definitely had an impact. The affected crops were sown in the very cold weather," said Mark Stevens, a scientist with the British Beet Research Orgaanisation.

The severe weather has also led to one of the latest fruit harvests in years. Major varieties of apple, including cox, are not expected to be in the shops until late September, said Adrian Barlow, chief executive of the trade body English Apples & Pears. "We don't yet know the long-term consequences of the winter. We have extremely cold weather which was good, but we have had trees with their feet in water for months and extremely windy conditions. But we are hopeful of a better harvest than last year."

However, the long winter could make for a succulent soft fruit crop, according to Harry Hall, Britain's biggest strawberry grower. "The crop is around three weeks later than average but the fruit has had extra time to grow so the quality is really good," said Hall, who expects to sell 7,700 tonnes this year.

Food prices are unlikely to rise significantly as a result of the poor British harvests, because these are determined by international trade. However, cereal food producer Weetabix had to halt production of some of its breakfast cereals as a result of the disastrous wheat harvest in April.

The full impact of the hard winter is only now being seen, according to the Department for the Environment, Food and Rural Affairs. Its latest analysis says the total income from farming decreased by £737m in 2012 to £4.7bn, with farmers facing both crop losses and higher costs to feed their animals.

Earlier this year the British Agriculture Bureau said flooding in the UK had caused £1.3bn in damage in 2012.

This week, the UN and the Organisation for Economic Co-operation and Development reported that environmental factors such as droughts, bad harvests and lack of land were likely to limit the growth of global food output. Agricultural production is expected to grow 1.5% a year on average over the coming decade, compared with annual growth of 2.1% between 2003 and 2012. Another widespread drought like the one experienced last year in the US and elsewhere could raise crop prices by 15%-40%, it said.