The creator of the Dukan Diet has suffered a humiliating legal defeat against a rival who accused him of being a dangerous fraud.
Pierre Dukan, who has been credited with maintaining the super-slim figures of women like the Duchess of Cambridge, her mother Carole Middleton and sister Pippa Middleton, lost a libel trial against Jean-Michel Cohen.
Dr Cohen had used an interview with a French magazine last year to link the protein-heavy Dukan Diet, which is followed by millions the world over, with heart problems and breast cancer.
Dr Cohen said only 'the slimming industry, doctors, pill salesmen, publishers, and newspapers' had really benefited from the diet after 'climbing on to the bandwagon of this fantasy'.
Lawyers for Dr Dukan had told a Paris court in May that such comments went beyond normal professional rivalries, and amounted to a character assassination.
But the Tribunal Correctionnel de Paris ruled yesterday that Dr Cohen was entitled to express his views under the French constitutional guarantee of free speech.
'This was a total defeat for Pierre Dukan,' said Dr Cohen's lawyer, Richard Malka.
The court even ordered Dr Dukan to pay 3,000 euros - the equivalent of £2,700 - to Dr Cohen for bringing an 'abusive' legal action.
The hearing did not judge the rival claims on scientific or health grounds, but solely in relation to the libel claim.
After the ruling Dr Dukan insisted that his methods were completely healthy and criticised Dr Cohen for ignoring 'medical ethics which forbid public criticism of a colleague'.
He said Dr Cohen's rival diet, which is largely based on calorie-counting and exercise, had failed to prevent the overweight population of France rising to more than 22 million.
Originally launched in France, the Dukan Diet has proved an Atkins-style sensation in Britain and the U.S., with millions following it in books, newspapers, magazines and on the internet.
Some 250,000 copies of the Dukan Diet book have been sold in the UK alone.
Asked for the secret of its British success in the UK, Dr Dukan replied that it was down to two factors. 'It works, and Carole Middleton,' he said.
Mrs Middleton and her daughters Kate and Pippa are all said to have followed the Dukan with spectacular results in the build-up to the royal wedding. Celebrity Dukan fans also include Jennifer Lopez.
In line with the relatively modest libel payouts available in French courts, Dr Dukan had sued for a modest £13,000 in damages.
But, now that he has lost, he risks wrecking the worldwide slimming empire which has made him millions.
The Dukan Diet aims to establish users' 'real weight' before helping them maintain it for the rest of their lives, concentrating at first on 72 high-protein foods including fish and non-fat yoghurts. Copious amounts of oat bran are also involved.
Vegetables and carbohydrates are introduced later on, but dieters must have one protein-only day a week for the rest of their lives.
Dr Dukan claims that 40 per cent of those who follow his diet never regain weight - compared with the usual figure of around 80 per cent.
The rival regime preferred by Dr Cohen allows slimmers to eat anything they like so long as they stick to programmes varying from 900 to 1,600 calories a day.
They can thus create good eating habits and build their own diets based on likes and dislikes - as well as avoiding anything which might affect their health.