Diet plans may come and go, but Weight Watchers is for ever. Now even the NHS can't get enough of it. As the organisation celebrates its 50th anniversary, Charlie Cooper asks, just what is it doing so right?

There's no point denying it any more – we are a nation of fatties. Two thirds of English men are fat. Considerably more than half of English women are fat. A third of little boys and nearly a quarter of little girls in England are fat. Just under two-thirds of adult Scots and well over half of Welsh adults are – you guessed it – fat.

By fat, read overweight or obese – or a body mass index over 30. The NHS keeps tabs on these things because they predict that a tidal wave of heart disease, cancer and other health problems associated with being overweight will hit Britain over the next generation.

It was perhaps not surprising then, that a few years ago, doctors turned to a new ally in their war on our expanding waistlines. Weight Watchers – best known to many for those rather kitsch 'before and after' adverts, usually featuring the happy weight watcher posing inside extraordinarily large trousers – is starting to be taken seriously as a cure for obesity.

More than 2,000 GP surgeries now refer people to Weight Watchers and since 2007, the NHS has handed over £4 million to the organisation to help their patients lose weight. Of all the diet plans the NHS recommends, it is the most successful and has the highest number of referrals. The UK health establishment appears to be saying one thing: the big trouser adverts were right all along.

Now in its 50th anniversary year, the world's biggest diet company has never been more influential. It has been a benign part of the cultural landscape for so long now that the thought of it being at the forefront of public health seems slightly absurd. To some, the abiding image of Weight Watchers will always be Little Britain's Matt Lucas in drag, screeching at a 'Fat Fighters' class as the loathsome Marjorie Dawes.

Fashionable diet plans come and go, celebrities swear by the benefits of this long- forgotten super-food, or that ancient Buddhistf exercise regime, but Weight Watchers has somehow endured – adapting to changing times and changing science, but at heart the same organisation that was founded in 1963 by Jean Nidetch, an overweight New York housewife.

In the UK, two million people use their services every year. Worldwide, the company brings in revenues of more than £1 billion per year, operates in 30 countries and employs more than 50,000 people. It is owned by the private equity firm, the Artal Group.

Greater success has brought greater scrutiny. Earlier this year, Weight Watchers was subject to an investigation by Channel 4's Dispatches. The programme criticised the fact that some of the studies used to back up Weight Watcher's scientific credentials had been funded by the company itself, and questioned whether long-term weight loss always followed short-term success. The company responded that they always disclose such funding and "in no way" influence the studies. Furthermore, they say, "the majority of those who lose weight with the help of Weight Watchers do not regain it." Meanwhile, in the US, low-paid team leaders have taken management to task over what they see as their meagre salaries, made all the more galling by the rumoured million-dollar contracts handed to celebrities such as Jessica Simpson and Jennifer Hudson to sell the brand. In the UK, the latest ambassador is the actress Patsy Kensit, who has been the darling of the Daily Mail ever since losing 14lbs with Weight Watchers. Executives are reportedly paying attention to their employees' concerns, and have hinted they will increase compensation.

But for an organisation that's been around for so long and spread into so many countries, Weight Watchers has relatively few naysayers.

It has been around for so long, in fact, that when the company started up, post-war rationing was a recent memory; Western populations were just getting used to the idea that they could spend their disposable income on extra food. Gradually, obesity began to replace malnutrition as the major diet problem in developed countries. In this respect, Weight Watcher's profits expanded with the waistlines of the Western world.

"Since the 1960s there's been a tremendous rise in rates of unhealthy weights," says Zoe Hellman, Weight Watchers' head of public health. "There is a need for healthy weight management programmes and that is a sad fact. Our average member is obese. These aren't people looking to be the skinniest people, wanting to look like a model, these are people that really need to change their behaviour and lose weight."

The NHS seal of approval has been a significant feather in the Weight Watchers cap, allowing them to claim that they offer "a public health service" – not just another fashionable diet plan.

"If you look at other fad diets, what you'll often find is that they've cherry-picked the science," Hellman says. "If, for example, you wanted to promote a peanut diet, you could probably find some study somewhere that helps to back up your approach. You could use that one study to say 'Look, it works, this is what we're going to do!'. What we do is look at the pool of evidence out there, which is consistently evolving. We take everything that is proven and known in the science and ask: 'What does everything out there point towards?'. That's how we achieve healthy, realistic weight loss, with realistic, attainable goals."

However, a significant part of its success is clearly down to a less noble goal. Many, probably most, people – and women in particular – want to lose weight, not for the sake of their health, but because big is not considered beautiful. It is no coincidence that Weight Watchers' early growth coincided with the era of Twiggy and the establishment of the idea that the most beautiful women were skinny women. Even today, although there are more fat men in the UK than there are fat women, 90 per cent of Weight Watchers members are women.

Nor are most people joining Weight Watchers today to keep their GP happy. The trigger to join usually comes from a "seminal moment" in someone's life, Hellman says.

"The proportion of people who are citing health reasons in general has been increasing," she says. "But for the majority, deciding to walk through the door seems to come from some sort of seminal moment: a new job, new partner, some sort of goal that people want to get to – a wedding, a holiday. But overall, people generally join Weight Watchers because something has triggered them to say, 'Right that's it – time to change'."

Lidia Pembroke, aged 57, lost four stone when her boyfriend left the country for a new job in the Falklands on a 27-month contract. "I needed something new to focus on and so I decided to start Weight Watchers. I didn't mention anything to him, as I thought I would use the time to really surprise him on his return."

For Beccie Grime, aged 41, who lost 75lbs, it was something as simple as turning 40 that prompted the change. "I wanted the second half of my life to be healthy and I'd been overweight since I was 20; it felt so good to know I was doing something positive to make the next 20 years better," she says.

Underlying the Weight Watchers' ethic appears to be an idea that fatness is more than just a health risk – it reflects some kind of personal failing that people want to exorcise from their lives. The Weight Watchers' experience, as well, goes beyond just eating differently or exercising more. For those unfamiliar with the routine, read on. Weekly meetings and a strong emphasis on community give the organisation the air of something between a breakfast club and a cult. It even has its f own vocabulary. 'Tracking' is the name for a weight watcher's constant monitoring of how many 'pro-points' they have used up. Every food has its own pro-point score, depending on its potential to cause weight gain.

Each member gets a bespoke allowance of points to use up every day, based on their current weight and their goal weight. 'To pro-point' is another new verb the company has added to the lexicon, meaning to assign a pro-point value to a food product. New members receive exhaustive indexes of what foods have what value. Pro-points can be gained by doing exercise, while fruit and vegetables are 'free' of pro-points. Ultimately, it's eat less, move more. But that's just the start.

After a week of dieting, weight watchers meet in a local public place (usually a community centre or school hall) and debrief. Sessions start with an anonymous weigh-in. The team leader, almost always a former weight watcher themselves, must be privy to the progress someone has made in a week, but they do not share it with the group unless given permission. Then follows a motivational talk in which the group discusses what went wrong and what went right this week. The team leader offers additional straightforward dieting advice, but their role is more than just advisory. Normally they are not professional dieticians, but people from the local community and they go so far as to give out their personal telephone numbers to group members and offer to be on call 24/7.

James Morgan, a 52-year-old church organist from north London, is the team leader of his local Weight Watchers group, which meets every week in a community centre near Notting Hill. A zealous weight watcher, he dropped four stone several years ago and now leads three meetings a week.

He runs his session with a benevolent intensity, surrounded by stalls of Weight Watchers products and in front of a display board covered in diet advice and, conspicuously, his own 'before and after' shots. The shirt he's wearing still appears to be slightly over-sized.

Having shepherded each member through the anonymous weigh-in, he addresses the group. He reminds them about the paramount importance of tracking their pro-points – how it is crucial if there is to be any "accountability" come weigh-in day.

"If you are one of those who hasn't lost this week, ask yourself: 'Why did I go over that particular day?' Remember, not all hunger is here…" he says, touching his stomach. "It's also here…" he touches his heart. "Was the cat sick? Were you stressed? Was there a problem with a family member?"

In fact, James identifies three types of hunger: hedonic hunger (which is "hunger in the head": things we don't need but want, like ice-cream); emotional hunger, better-known as comfort eating; and finally, plain old biological hunger.

After the talk, there are prizes for this week's biggest 'losers'. A 'Silver 7' is awarded to a member who has lost seven pounds. Longstanding members can measure their pedigree by the number of 'Silver 7s' they have accumulated. One helper at the session who has recently received her eighth 'Silver 7' is a figure of particular reverence. When a member reaches their 'goal weight' they become a Gold Member – entitled to continue coming to Weight Watchers meetings for free, so long as they submit to five weigh-ins a year to check they don't exceed their goal weight by five pounds. If they do, they have to start paying for the service again.

The very fact that someone would want to carry on coming to meetings that do not amount to much more than a get-together and brief chat is testament to the real secret to Weight Watchers' success. It is more than a diet: it's a lifestyle.

"You have to live this," James explains after his members have left for the evening. "That is why other diets don't work." He continues: "You make a lot of friends. Community is the vital aspect". He has no qualms about the pastoral element of his role. "People tell me the most personal things," he admits. "If they're going through a rough patch, I know about it."

The prizes are "a lot like being back in school," he says. "They are a big helping hand. When you were seven, you loved the encouragement of getting a gold star and people still love the recognition at 47. But it's just a gentle motivator – not something we tie people down to."

As easy as it might be to be cynical about this kind of thing, it is done, in James's case at least, with the sincerest of motives. He genuinely cares about his charges, knows all of their names and badly wants them to lose weight. "I know how much happier and healthier it made me and I suppose it's a desire to help other people that made me want to become a leader after that," he says.

James happily admits that Weight Watchers changed his life and thousands have said the same. For some, it moves beyond its original purpose and becomes a way of life – a community of friends, confidants and well-wishers.

But it's not for everybody. Some who try it find it oddly disempowering, and the idea of weekly 'accountability' to the dreaded weigh-in a touch authoritarian. Much is said about the community ethos, but a community that starts charging its members if they sneak over a certain weight is not one that some people will feel comfortable belonging to for long.

However, people find time and again that, like it or not, it works – enough that doctors are beginning to trust in its methods and prescribe a 12-week dose of Weight Watchers to the overweight and obese. That may be a useful tool in the fight against the obesity crisis. For the hard-line pro-point trackers, though, 12 weeks will never be enough. For them, Weight Watchers was always about more than just dieting.