Slimmers, rejoice — those forbidden sweet treats or extra bags of crisps may no longer be off-limits. Scientists advising the Government say that the calorie counts used as the basis of diet plans and healthy-eating advice for the past 18 years may be wrong.

According to a draft report by the Scientific Advisory Committee on Nutrition (SACN), the recommended daily intake of calories — currently 2,000 for women and 2,500 for men — could be increased by up to 16 per cent, suggesting that some adults could safely consume an extra 400 calories a day (equivalent to an average-sized cheeseburger, or two bags of ready-salted crisps).

The committee, made up of some of Britain’s leading nutritional experts, says that its report provides a much more accurate assessment of how energy can be burnt off through physical activity.

However, health campaigners and consumer experts warned that the Department of Health and the Food Standards Agency (FSA) could seek to “sweep this report under the carpet”, as it could send out mixed messages in the middle of an obesity epidemic.

Ministers are considering the introduction of new food-labelling schemes that would highlight the calorie content of foods relative to guideline daily amounts (GDAs).

Industry sources expressed concern that revising figures and estimates on which the GDAs were based could cause confusion among consumers and mistrust of scientific advice. The FSA has been evaluating for two years new methods of labelling, including a “traffic light” scheme to colour-code unhealthy food.

Existing guidelines on energy intake required for good health have formed the basis of food labelling and dietary advice from doctors and nutritionists since 1991. If the committee’s proposals are accepted some foods would be upgraded to a healthier rating.

The draft proposals, seen by The Times and The Grocer magazine, are due to go out for a 14-week consultation before final recommendations are made. The report comes two weeks after the Government’s chief drugs advisor was sacked by the Home Secretary for “crossing a line” by publicly criticising existing policy.

About 60 per cent of British adults are overweight or obese, with growth in the nation’s waistlines being blamed on sedentary lifestyles as well as excessive eating. The cost of overweight and obese individuals to the NHS is estimated to be £4.2 billion a year and the Department of Health has pledged to cut levels of childhood obesity partly through its £375 million “Change4Life” strategy.

Tam Fry, of the National Obesity Forum, said it was a “dangerous assumption” to say that adults could safely consume an extra 400 calories a day. “This is not a green light to eat yourself silly,” he said.

The last significant study on energy use, carried out by the Committee on the Medical Aspects of Food and Nutrition Policy in 1991, was based on observational studies, with students being shut in a room for a week having their breathing measured, a method prone to underestimating “normal” levels of physical activity.

SACN assessed studies using the Double Labelled Water technique, which measures how much carbon dioxide the body has produced converted into equivalent values of energy.

Adam Leyland, editor of The Grocer, said: “The ramifications for the industry are significant, to say the least. All the UK’s labelling schemes, including GDAs and traffic lights, are based around the 1991 energy report.”

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