Tesco and other companies that boycotted the new national food labelling scheme are under attack after research showed they have left shoppers confused.

Polling of 600 people for Which? ranked Tesco's way of highlighting levels of salt, sugar and fat in processed food the least helpful of four rival options.

People most easily understood the traffic light labelling system from the Government's Food Standards Agency (FSA) which signals the nutrients with different colours, red meaning bad and green good.

The survey undermines the justification for the boycott by Britain's biggest food retailer of a measure regarded by campaigners as a crucial weapon against obesity, estimated to kill 35,000 in Britain every year, a factor in cancer and heart disease.

It is also another public relations blow to Tesco, which has been struggling to counter claims that it bullies local authorities, suppliers and small rivals, harms the environment and cuts animal welfare standards.

The Government's traffic light scheme, introduced this year after the FSA surveyed 2,736 consumers, has been adopted by Sainsbury's, Asda, Waitrose and the Co-op.

Advocates say it is simple to understand, and initial sales figures from Sainsbury's suggest its introduction has led to plummeting sales of junk food.

But many big industry players, including Tesco and Morrison's, are rejecting the system. Tesco's scheme is based on how much each food contributes to calories, sugar, fat, saturated fat and salt as a percentage to an individual's GDA (Guideline Daily Allowance). The labels have colours but they do not change according to the levels of nutrients.

Campaigners say shoppers do not have the time or inclination to add up percentages for products as they wheel their trolleys.

Tesco claimed the FSA scheme would confuse customers because some product were healthy despite having a red light, including orange juice, which is high in sugar.

Many big manufacturers of processed food, such as Kellogg's, Nestlé, Danone, Kraft and Pepsi, intend to introduce a version of the Tesco GDA system, without colours.

A Tesco spokesman said: "While we understand that traffic lights may give a simpler initial impression, customers have told us that our system is more useful in taking practical steps towards a healthier diet.

"Sales data demonstrates that when nutritional signposts are added many shoppers switch to products which are lower in salt and fat. Traffic lights may never produce these results."