Critics have lined up to accuse Fern Britton of lying and cheating after it was revealed that she didn't tell the public about having surgery to help her lose weight. Give the woman a break, says Katy Guest

Question: which news event that made headlines last week was described by one commentator as "the most sickening act of deception I think I have ever come across"? Was it a) the revelation that a Tory MEP had channelled more than £400,000 of taxpayers' money into a family firm? b) Robert Mugabe's ongoing machinations to fix the forthcoming election in Zimbabwe? Or c) news that the female presenter of a daytime television show had been through an operation to combat her obesity, but hadn't told anyone?

As usual, the clues are in the columnists. When the news broke in the News of the World on Sunday, that paper's opinionist, Carole Malone, decided that the person responsible must hate herself. The following day, Anne Diamond disagreed. "Only ignorant, unfeeling snobs would call [her] a cheat," she wrote. That must have made Amanda Platell feel pretty rotten when she blamed the miscreant on Tuesday for almost single-handedly causing the national obesity crisis. And Sue Carroll must have felt even worse on Wednesday when she accused the woman of letting down the NHS.

Even sadder, poor Kelvin MacKenzie will be kicking himself for what appeared under his name. "It's not like me to be unkind," he began, inexplicably, "but I wonder what the doctors used for Fern Britton's gastric band. The inner tube of a tractor tyre comes to mind."

Yes, this is the revelation that Fern Britton, the blonde gigglepuss who co-presents ITV's This Morning, has lost five stone after having a gastric band fitted two years ago. They are not cross with her for losing the weight, her critics insist. Not angry – just disappointed. Ms Britton didn't tell her fans about the operation. She allowed them to believe that her new look was down to healthy food and cycling. By extension, she has lied to and cheated her public. This means she has gone from cuddly-wuddly national sweetheart to despicable villain overnight.

On This Morning on Monday, an uncharacteristically stony-faced Ms Britton explained herself. "Two years ago I had a gastric band, and do you know what? I did it for me because I wanted to," she confessed. "I didn't feel that I had the need to ring around and let everyone know. In fact only five people knew, including my husband obviously. I didn't expect such enormous interest in body image and perception."

Even for a legendary optimist, that is disingenuous. Or perhaps Fern just doesn't read the papers. If she did she would know by now that her body, its image and the perception thereof are public property and that the people of Britain think of little else when they are deciding whether or not they ought to live on doughnuts.

So said Amanda Platell in 2005, when Ms Britton was photographed in a bikini. She wrote that "Fern's weight is an issue – a big issue ... there comes a point when an occasional self-indulgence becomes greed." She blamed the size-22 presenter for her decision to "endorse" a lifestyle that costs the NHS £500m and causes 30,000 deaths a year and told her to pin the photos to her fridge door and use them as "a wake-up call, the beginning of a new start". She didn't instruct her to alert the media the minute she had decided how to cure her contemptible gluttony, but that, surely, was implied.

Of course, criticism of Ms Britton as too fat, too thin or too happy or unhappy in her own skin is only what any woman must expect when she steps in front of a camera. But behind the usual pointing-and-laughing, there's a deeper resentment. Critics have repeatedly come back to two words: lying and cheating.

She appears in an advert for Ryvita, in which her body is digitally replaced to comic effect by that of a waifish model. "My daughter (14) seemed to buy into the fact that Fern had gone on the Ryvita diet," one viewer complained. "I told her she couldn't drop so much weight on Ryvitas alone but impressionable teenagers want to believe that it can be done." (Actually, you can drop a lot of weight on Ryvitas alone, but you see her point.)

Previously, Fern's husband, the television chef Phil Vickery, had let it be known that she had been eating more healthily and cycling (she took part in a charity bike ride in India and was clocking up 60 miles a weekend during training). Unfortunately, all that is now seen as just a clever smokescreen.

"Next time I have a facelift or haemorrhoids or something, I will ring the Sunday papers straight away," Ms Britton joked:. (Thanks, but you can give us a miss.) Her point is that she wouldn't come clean in public about any other serious medical procedure, particularly one that still carries substantial risks. But her critics complain that she is seen as a "role-model" for women because of her weight, and so she has a responsibility to share her battle with it. "Her size is integral to her identity and public appeal," argued one.

The accusations of "cheating", however, are more complex. Is stomach surgery a wimp's way out? Appearing for the defence, Anne Diamond, who has also had a gastric band fitted (she had to leave the programme Celebrity Fit Club in disgrace as a result), said: "So-called experts say there's only one way to lose weight, and that's through deprivation. It's rubbish, and I say well done Fern for finding another way."

But that's not what other patients say. Last week, a London GP who underwent stomach surgery earlier this year told The Independent on Sunday about how hard it is: the fear, the risk, the unending deprivation, the desperate food envy, the pain caused by eating just a teaspoonful over the maximum one cup a day.

The sad truth is that there isn't really any way to cheat yourself thin. But try telling that to the mass media. Ms Britton herself was the subject of a "celebrity diet secrets" exposé that appeared, with unfortunate timing, in Yours magazine on Monday. But she is only one of many famous women whose weight loss has been used to sell magazines with the promise that "you too can drop six dress sizes by eating whatever you like".

"Emma Bunton lost her baby weight by eating chocolate," read an amazing story in the News of the World, the paper that first revealed the truth behind the Britton diet. Other papers revealed that the Spice Girl had signed up to the "no white food" diet, which bans everything from pasta and potatoes to milk. But Ms Bunton herself said that she had in fact shed the weight by spending several hours a day training for Strictly Come Dancing.

"She loves to scoff cookie dough, buttered popcorn, onion rings and pretzels dipped in salsa sauce. So how on earth does Katie Holmes, 29, keep a stunning figure?" asked a headline in the Daily Star recently? Well, there's a clue in the fact that she ran last year's New York Marathon – and that she probably doesn't really scoff cookie dough.

Various magazines swear that Jennifer Aniston stays slim by eating baby food. Or is it by sprinkling mustard seeds over everything she eats? Mel B lost four stone by having sex. Or by a training regime of "boxing, weights, circuits and sit-ups for two hours each day", if you read the small print. And if Victoria Beckham and Kate Moss ate everything they are said to eat in order to stay thin (strawberries, prawns, white tea with honey, probiotic yoghurt, Chinese fat-digesting teabags, grapefruits ...) they'd be enormous. Even Gwyneth Paltrow recently dismissed claims that the weight fell off thanks to a raw food diet. "I hated the stomach roll, the back fat and the post-pregnant butt. And it was so hard to get rid of. Especially the final 20lb."

Fern Britton, for right or wrong, kept her medical record to herself. She had medical help with the willpower. She may even have misled her fans. But is this an argument over lying and cheating about quick and easy ways to lose weight? If it is, Ms Britton isn't the only one with a case to answer.