City analysts have found that within two months of Tesco introducing nutrition labels in April last year it was selling 26%-37% fewer prawn mayonnaise sandwiches and ready meals such as steak rosti bake and chilli beef noodles.

Separate figures released by Sainsbury this weekend show that sales of chicken madras have fallen 40% since it started its labelling scheme 15 months ago. Other ready meals have also seen sharp declines.

The government’s Food Standards Agency (FSA) has been negotiating since 2004 to persuade food manufacturers and supermarkets to adopt its “traffic light” scheme for packaging. This uses colour coding to indicate levels of salt, fat and sugar per 100g and provide consumers with information about the nutritional value and potential health risks of their diet.

The scheme has been accepted by only a few chains. Waitrose took the first steps towards it last month with the introduction of traffic lights on sandwiches, but says it is too early to see the results.

The falls in sales of some foods suggest, however, that the supermarkets’ own schemes are having a positive effect. The result may force producers to improve the nutritional value of processed foods.

Analysis of Tesco sales by Jeff Stent, a food industry specialist at Citigroup investment bank, suggests some foods, for example pork pies and meat pasties, simply cannot be made into healthy products. A woman eating a pork pie could be exceeding her daily recommended fat intake by 20%.

Stent’s report says: “The scope for improving the nutritional content of savoury pastry is both . . . limited and costly. Savoury pastry products may face a very challenging future.”

He compared sales at Tesco, Britain’s biggest supermarket chain, in the eight weeks before nutrition labelling was introduced with sales in the eight weeks afterwards.

The labels on the front of packets tell people what proportion of their “guideline daily allowance” (GDA) of sugar, fat and salt is represented by each item.

Sales of salmon en croute ready meals fell by more than a third once shoppers discovered that a pack contained 91% of the daily saturated fat allowance. Croissant sales fell by 8%, while stores sold twice as many low-fat, low-salt egg-and-cress sandwiches.

Sainsbury was the first supermarket to introduce explicit front-of-pack colour-coded labelling 15 months ago. Following recommendations from the FSA, products were given red, amber and green codes to indicate fats, saturates, salts and sugar.

According to the chain’s figures released this weekend, sales of lower-fat foods have jumped. Spinach and ricotta cannelloni, and carrot and coriander soup from the Be Good to Yourself range are up by 142% and 123% respectively.

“We have seen significant changes in customer behaviour,” said Justin King, chief executive of Sainsbury. “It is influencing our thinking when we are developing products. If we make our products more healthy, more people are buying them.”

A spokesman for Unilever, which owns 28 UK brands including Hellman’s, Pot Noodles and Marmite, has introduced low-sugar and low-salt products, but says most consumers prefer “standard” versions.

Associated British Foods says: “There is no such thing as bad food, only bad diet. We support the consumer trends of convenience, indulgence and health. A healthy balanced diet will span all three of these — for example a pork pie served with salad.”

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