Britain is facing its worst harvest for at least 40 years as 30 per cent of the country's grain lies in waterlogged or sodden ground. Hilary Benn, the Rural Affairs Secretary, is expected to give the go-ahead today for farmers to salvage what is left of their crops by using heavy machinery on wet fields.

European Union rules ban farmers from using combine harvesters on wet land to protect soil quality. Those who flout the ban can be prosecuted. The exemption is expected to last for about three weeks.

The poor harvest is unlikely to lead to a rise in the price of bread, cakes, biscuits and flour, however. Gordon Polson, director of the Federation of Bakers, said that although much of the milling wheat was of a poor quality it could still be used for bread and flour.

He said: "The poorer wheat means it has less protein, but manufacturers can add gluten to ensure the proper quality for making bread. We are not happy and we may still have to import some milling wheat, but no one is talking about price rises for bread."

The harvest has been most badly affected in the North East, especially Northumberland, North Yorkshire and Co Durham, where the heavy rainfall and flooding have meant that on many farms less than 50 per cent of the wheat has been harvested.

Farmers stand to lose as much as £30,000 each because much of the grain in the soil is suitable only for animal feed. The price of this lower-quality grain has dropped from £120 to £100 a tonne in a month; best milling wheat is valued at £140 to £150 a tonne.

There is also concern about next year's harvest. Unless farmers clear this year's crop within the next two weeks many will be unable to prepare the ground and plant new seed.

About 200 acres of wheat are waterlogged at Tom Neill's farm at Mindrum, Northumberland, near the Scottish Border. He said: "I am 64 and this is the worst harvest I've experienced in my 50 years of farming. I am not certain even if we get permission to use our machinery on wet fields whether I will be able to salvage half of my crop. Unless we get a dry spell it will be too late. It's very depressing and I could lose as much as £20,000 or £30,000."

Industry experts are revising their estimates for the total wheat yield. Early forecasts were that it would be as much as 16.5 million tonnes, but this could be cut by 10 per cent.

Guy Gagen, chief arable adviser at the National Farmers' Union, said: "This is the most difficult harvest for at least 40 years. Farmers say it compares to 1968, which was very similar, with heavy rain throughout the summer. We just need a break in the weather. If we get that for five to seven days farmers can recover."

Simon Ingle, a wheat trader at Grainfarmers, which buys 20 per cent of home-grown wheat, said: "We are still hoping yield will be up, but this harvest is the worst I have known since 1982. Many crops will be lost and the price of this lower-quality wheat may continue to fall."

Mr Ingle added that Britain was the country worst affected in Europe and that most European countries completed their harvests last month.