A survey of major retailers shows that two of the country's leading supermarkets - Tesco and Asda - recorded a 20 per cent increase in February sales, compared with January.
Tesco's organic food buyer, Andrew Sellick, said: "The upsurge was nothing short of phenomenal, and it is the awareness of the GM issue which has pushed the sales."
Sainsbury's organic sales were worth pounds 6.7m last month, compared with pounds 5.8m in January, a 15.5 per cent rise.
Marks & Spencer, a medium-sized food retailer, said its increase was "more than 100 per cent." Iceland, which has a small number of organic frozen foods, had a 42 per cent increase.
Organic vegetables, fruits and cereals are grown without pesticides or artificial fertilisers - natural methods such as crop rotation take their place - while livestock and poultry are raised without intensive farming techniques. All organic food is guaranteed non-genetically modified. Farmers must spend two years converting their land before it can be organically certified.
Organic carrots or apples do not look as perfect and regular as the products of large-scale agribusiness. Production techniques are also more laborious and the costs greater.
Seventy per cent of the organic food sold in Britain has to be imported. Less than one per cent of farmland here is organically managed, by fewer than 2,000 farmers. Germany, Austria, Sweden and Denmark are all aiming to have 10 per cent by next year.
Overall the market is growing at 40 per cent a year. Tesco, Britain's largest food retailer with more than 15 per cent of the UK grocery market, has more than 200 organic lines from potatoes to yoghurt.
The company sold pounds 35m of organic food in the 12 months to 1 March, more than double the amount in the previous year. In the next 12 months Tesco says it will sell produce worth pounds 100m.
UK sales of organic food as a whole have risen from under pounds 100m annually in 1993 to pounds 260m in 1997 and about pounds 400m last year. The pounds 1bn barrier could be broken next year, according to Simon Brenman, manager of producer services for the Soil Association, the principal organic food and farming pressure group.
Senior food industry figures said February's remarkable extra sales surge was caused by the GM controversy.
One of the most striking instances of sales growth last month was in organic baby food. Baby Organix, Britain's only organic babyfood manufacturer, which supplies Tesco, Sainsbury, Safeway, Waitrose and Boots, had its best month in February: its sales were 24 per cent higher than the month before.
"I know this is down to the GM controversy," said the company's founder and managing director, Lizzie Vann.
"People feel they won't compromise with their babies. If a woman's pregnant and she's given up drinking and smoking, when she gets the baby she's not going to start fooling around with its food."
Calls to the company's freefone helpline jumped from 300 a week at the start of February to 800 a week now.