Amid the avalanche of seasonal diet books the Government's watchdog on new treatments today adds another to the pile - with the first official advice on what constitutes the ideal weight-loss diet.

It warns that crash dieting may be dangerous and recommends cutting down by no more than the equivalent of one Mars bar and one Danish pastry a day as the most effective way of losing weight.

In competition with titles such as The Duvet Diet and The Wine Diet, the latest publication from the National Institute for Health and Clinical Excellence is unlikely to be a best-seller. But the "Guidance on the prevention, identification, assessment and management of overweight and obesity in adults and children" is likely to have a greater impact on the the 27 million people in England estimated to be overweight or obese.

Published today, it is the first guidance to combine recommendations on the prevention of obesity with advice on its treatment. It warns that "unduly restrictive and nutritionally unbalanced diets" are ineffective and says the ideal diet is one which contains 600 calories a day less than the individual burns in energy. This is most likely to result in "sustainable weight loss".