ch require a lot of water, will be grown in Britain, experts fear.

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Efforts to make up for shortages by importing vegetables would increase concerns about the environmental impact of transporting food.

A spokesman for Which?, the consumer organisation, said: "If the shortage resulted in vegetables being brought from overseas, that would not be a good thing in terms of 'food miles' and a drawback for the -environmentally conscious consumer."

Tesco is currently monitoring the situation, a spokesman said.

Many farmers have already changed to cereal crops that are sown in the winter and are less sensitive to water shortages.

Last month was the warmest April since records began almost 350 years ago. It was also among the driest.

Parts of Britain received no rain while East Anglia, where much of the country's vegetable cultivation is centred, had only 2.2 mm, 5 per cent of the average rainfall for the month.

The dry weather means that seeds are not germinating or are dying because of a lack of moisture.

Growers, who complain that they are already experiencing "drought conditions", warn that even prolonged rain now will come too late.

Where possible, fields are being irrigated, months earlier than usual. However, with only 60 per cent of vegetable fields having irrigation systems, some farmers have called a halt to planting.

Stephen Francis, managing director of Fen Peas, which grows 4,000 acres of peas in Lincolnshire for supermarkets to freeze as their own brands, described the situation as "very serious" and explained that, in the pursuit of moisture, he had been forced to plant seeds four times deeper than usual.

"We've got drought conditions," he said. "We've had no rain since the first week in March and have been struggling to get these crops to grow. Even if we get rain, it will be too late.

"We had a poor year last year, so there is not going to be enough stock around. There's going to be a shortfall and there are going to be shortages."

Tim Mudge, from the Processed Vegetable Growers' Association, said: "Everything that grows needs moisture to do so. There has been a change in weather patterns and we will have to analyse them and may have to change what we do with our crops."

Britain is the biggest pea producer in Europe, growing up to 150,000 tons annually.

British farmers grow about a million tons of carrots a year and 40,000 tons of leeks. A total of 112,000 tons of brassicas are also grown. About two thirds of these are cauliflower and cabbage, with the rest made up of broccoli and Brussels sprouts.

Carrot growers, who store their vegetables on the ground under straw during the spring, are reporting soil temperatures of up to 7 C (44 F) - more than three times warmer than the average a decade ago. Martin Evans, the chairman of the British Carrot Growers' Association, said: "It's been a very stressful spring. We're expecting lower yields which we'll start to see in mid-summer.

"Climate change is bound to have an impact. We'll have to adapt and look for alternatives because of the risks involved in growing some vegetables."

Some leek farmers predict that when they harvest their crops, yields will be down by as much as 30 per cent.

Colin Bayley, a spokesman for the Leek Growers' Association, said: "Establishing crops is a problem. Without rain, they die."

Richard Hirst, who cultivates peas near Great Yarmouth in Norfolk, for the food manufacturer Birds Eye and who is chairman of National Farmers' Union's horticulture board, said: "There may well be some changes to the crops we grow. There is research going on to see if we can give our plants some drought tolerance."

Alastair Ewan, a director of the Brassica Growers' Association and a farmer in Cupar, Fife, said: "People are having to irrigate, which you don't expect to have to do until well through June. Rain that falls now won't help to replenish water stocks."