The first thing I notice, on stepping into Dr Pierre Dukan's consulting room in a beautiful old building in an expensive part of Paris is the huge painting of a naked woman spread-eagled with everything on show on one of the walls. This is what greets the people he treats for obesity and I wonder what they make of being confronted by this slim, sexy woman. Is it the fine art equivalent of the picture of the supermodel taped to the fridge? It seems so absurdly out of place that every time I catch sight of it I have to try to stifle childish giggles.

If your only reminder of Christmas is a tightened waistband, you may be considering starting the Dukan diet. Devised by this French doctor and nutritionist, it got a fresh boost last year, popularised by the news that Carole Middleton used it to lose a few kilograms before her daughter's wedding to Prince William in London. Gisele Bundchen, Jennifer Lopez and Penelope Cruz are all said to use Dukan's method.

So far, so faddy, but even eminently sensible people, such as the BBC broadcaster Jenni Murray, have raved about it, having started it after Dukan appeared on her radio program Woman's Hour.

Dukan published his first book in France in 2000, and in Britain in 2010. It has sold 7.5million copies worldwide and he estimates about 12million people have tried the diet. But despite ongoing sales, the scales may be beginning to tip against him.

In July, Dukan lost a slander case against Jean-Michel Cohen, a nutritionist who told a French magazine the diet could cause ''serious health problems among certain patients, such as a strong rise in cholesterol, cardiovascular problems and breast cancer''. A survey of 5000 people in France who had followed the diet found 35per cent regained the weight they lost within a year and 80per cent regained it more than four years later. In November, the British Dietetic Association named the Dukan diet as one of the worst ''celebrity diets'' of the year.

The nutritionist also faced criticism last week when he published his latest book, An Open Letter to the Future President, which included 120 proposals to tackle obesity in France, one of which was to award extra marks to Baccalaureate students who either remained within a normal body mass index (BMI) weight, or lost weight over the course. The French health ministry issued a statement saying it was ''astonished at Dr Dukan's strange proposal that is unknowingly physically discriminatory''.

''Ah, this one!'' he says, leaning across the antique desk in his office. ''People say 'This is discrimination!'''

He laughs, and I think he seems to be enjoying his moment of notoriety. Isn't it discriminatory? ''Do you see it as discrimination?''

It encourages people to judge others on how much they weigh. ''I don't judge,'' he says. ''It's not a diet. It is saying 'I am with you'. The discrimination is already at school. If you are overweight, everyone looks at you and you feel it.'' He says he would only ask overweight students to ''slowly'' lose 2kg during the two years, and it would include a practical program about cooking and preparing food, and reading nutritional labels. It is only one of his ideas; the others include more food education and getting food companies to change their recipes to become low-fat and no-sugar (he's a big fan of artificial sweeteners).

He wrote the book, which he sent to this year's (French) presidential election candidates, because he calls obesity this century's ''biggest serial killer''.

''I have 42 years of experience, and I have travelled a lot, and I discovered that my diet makes a lot of people happy - but I think it's not enough,'' he says. ''In France we have 22million [overweight people] and the economic model of the West is built on a lot of obesity - food production companies and pharmaceutical companies who treat people for diabetes and heart disease. So I say to the future president, we have to do something. France could become the first country to help its people lose weight.''

Like what? Would he like to see higher taxes on fatty food?

''Yes. Like tobacco,'' he says. ''We know it is not good for us, and we pay a lot. We can do the same for snack foods ... It is not necessary to sell poison to be rich, you can sell the same products but lower in fat and sugar.''

France, though, looks positively skinny compared with Britain. Eurostat, the European Union's statistics office, recently released the results of its study of obesity levels across 19 countries. It found British women were the most obese in Europe. Nearly a quarter of British women - 23.9per cent - and 22per cent of men were considered to be obese. France had one of the lower levels, with 12.7per cent of women and 11.7per cent of men.

''I think America and Britain have a different culture from France,'' he says. ''They discovered marketing and consumerism before France. I think we resist a little bit more - it comes from the culture, it's not genetic. Sugar is more present in America or England than it is in France. I think there is an addiction to sweetness. We have a tradition of cooking, we like to eat, sitting together, not in front of the TV. When you are in front of the TV you are in distress and you... '' He breaks off to make a shovelling-food-into-mouth movement.

Dukan is a charming man, with a twinkly smile, who speaks English with a thick French accent. Born in Algeria to French parents - his father worked in clothing production, while his mother raised the children - he wanted to be an artist, but was also drawn to medicine. ''My mother said if you are a doctor, you can paint and you can buy paintings. But if you are an artist, you will never do medicine. Now I like to spend time helping painters.''

The Dukan diet started 40 years ago with a comment to a patient he saw at his GP surgery who wanted to lose weight. He asked Dukan to devise a plan with the only restriction that he wouldn't have to give up meat. ''All right,'' Dukan told him, ''eat only meat, as much meat as you want for five days.''

When the patient came back the following week, he had lost almost 5.5kg and Dukan became convinced that a high-protein, low-fat and low-carb diet was the key to losing weight. He had been specialising in neurology but ''I stopped because it was too hard for me! I felt I was better in this type of illness.''

Dr Robert Atkins had already devised a similar plan in the US, though he advised people to banish carbohydrates and eat as much protein as they liked. ''It was interesting because it was different from the low-calorie diet dogma,'' Dukan says. ''And people lost weight, [but it contained fat and] fat is not good for health. But it opened a door. Counting calories was not easy and efficient.''

The diet Dukan eventually devised involves four stages, each sounding like some sort of military strategy. The first phase, ''attack'', involves eating only protein for between one and 10 days. The next, ''cruise'', introduces vegetables, alternating protein-only days with days where a list of vegetables can also be eaten; he recommends the dieter stays at this stage until they have lost all the weight they want to. ''Consolidation'' comes next, with more foods, including wholemeal bread, cheese and fruit, added in controlled portions, and one protein-only day a week. Finally ''stabilisation'', where the dieter can eat foods from Dukan's list, with one protein-only day, for the rest of their life. Tablespoons of oat bran are eaten every day to fill you up. His inclusion of ''celebration meals'' (two per week, where the dieter can eat whatever they like) is what stops people giving up, he says.

It is charmingly French in places. Dukan helps his dieters with warnings about the traps that could befall them - ''if you are invited to a friend who loves serving vol-au-vents as appetisers, then remember to fill yourself up beforehand.'' He tackles questions such as ''are you allowed mustard?'' and what to do when someone offers you a glass of champagne (make sure you are carrying a full glass of sparkling water at all times should this be a danger). His list of allowed protein includes rabbit, and lambs' and calves' tongue - perhaps not the kind of meat that will find its way into most Dukan followers' protein days.

There are outlandish claims that his high-protein diet combined with a lot of water can ''flush out'' cellulite, but he is also a firm believer in home-cooked food over processed, wholemeal bread over white, and ''prescribed exercise'' - at least 30 minutes of walking every day, going up to an hour each day. But still, here is a man who has grown rich - ''I am rich because I sell a lot of books, but it's not my drive'' - by unleashing yet another diet on the world. Does he ever worry that he is helping to fuel an unhealthy obsession with the way we look, an unnatural desire to be thin? ''No, I don't think so,'' he says. ''I think the real problem is to be too overweight. There is a small part of the population, young, nice-looking women who can be obsessed. I want to help people to lose weight.''

He refuses to see a link between the diet industry and the pressures it can pile on some women (and men). I can't tell if he's being disingenuous, or if his view really is that narrow - the patients he still sees a few days a week at his clinics between his writing and press engagements are obese and their weight is a serious health risk; he doesn't see the millions of women (and men) who take up diets when they don't need to. Is his ideal woman thin? ''No! I think it's better to be a woman with some curves. It's more natural.'' He smiles. ''My wife, she is too thin.''

When I ask about the study that showed 70per cent of Dukan dieters had put all their weight back on within three years, he says it ''was not really scientific''. His own figures, he says, show 25per cent of people have success using the books, which rises to 40per cent if they use the Dukan online coaching service. ''We are going to try to make it better and better. It's not easy. [People fail] because we live in a civilisation where it is very difficult. Before, we lived like hunters and with natural food. It was not easy to find food, and we had to make a lot of physical effort.

''Before the war, obesity was not a problem. In France, in 1960, there were half a million people who were overweight; today we are 22.5million overweight. It coincides with economic growth. You can see it happening in China. Foods were made with a lot of sugar, salt and fat, combined with marketing and advertising. People don't move so much.'' Restrictive diets have been accused of leading to malnutrition, heart disease and bowel disorders.

The Dukan diet will certainly help you lose weight in the short-term, spokesperson for the British Dietetic Association Ursula Arens says , ''but it is a very unbalanced diet and not something you'd want to see someone going on for three months or more. When you restrict access, you limit variety, and long-term nutrition science supports eating a variety of food groups.'' I raise Cohen's concerns that the Dukan diet could lead to heart disease and breast cancer. ''I have been working on this for 42 years and millions of people have done this diet,'' Dukan says. ''I've never heard of anybody's health suffering.''

Dukan is 70, looks incredibly well and appears to have the energy of a much younger man. Is he on the diet? He shakes his head. ''After Christmas sometimes, but not this year because I have too much work to do and I have no time to eat.''

The Guardian