As far as the management at Gauthier Soho is concerned, if you eat in their restaurant, you pay the bill. That will sound totally reasonable to the vast majority of the restaurant-going public, but for a growing number of food bloggers, tweeters, Instagram posters and Yelp reviewers, there is such a thing as a free lunch.

Online food writers who post reviews and pictures of restaurants they visit aren't new and some, such as The Critical Couple, The Lambshank Redemption and Tea Time in Wonderland, are well respected and attract thousands of readers.

But restaurateurs across the country are increasingly noticing something more sinister than a smartphone picture of a pretty pea soup or finely cooked fillet steak.

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In hushed tones, restaurant managers use the words "blackmail" and "bribe" to describe occasional bloggers who hand over their cards to the maître d' on arrival in the hope of a "little something extra", and others who brazenly email in advance requesting a "review meal" in exchange for a "positive" judgement online.

The latter, is allegedly, what food blogger Paul Turner of the Hungry Londoner did last week, when he emailed high-end independent restaurant Gauthier Soho in central London, apparently offering the possibility of a "positive review" if the £150-a-head establishment could "arrange a review meal". It would be, he continued, an "ideal promotion".

The hungry blogger – who by day works in the IT industry – hadn't counted on the fighting spirit of the restaurant's head of marketing, James Lewis, though. It was Turner who was promptly "publicised" on Twitter by Lewis, who posted his (at the least) ambiguously worded email, including his mobile number and email address, in a rant against what he calls "food blaggers".

Lewis has since had his Twitter account suspended and Turner has labelled the post an attack on his privacy, but the restaurateur is forthright in defence of his action.

He tells The Independent: "There's been an ugly development in recent times that I call the food blagger, which is someone who uses the food blog as a platform to gain free stuff under the disguise of a review. I can put up with most of this sort of stuff, but what really irks me is what I see as somebody trying to get a free meal in return for a guaranteed positive review... It's a bribe, basically."

The rise of this new type of more demanding food blogger is something that's not gone unnoticed by traditional restaurant critics, including The Independent Magazine's John Walsh, who, like all of this newspaper's critics, pays for his meals.

"The profile of food critics has grown in the last decade and now every foodie with an itch for self-expression wants his own column," says Walsh, who was named restaurant reviewer of the year in the latest Guild of Food Writers awards. "Hence the rise of bloggers and people who write reviews for the TripAdvisor and Yelp websites. Some are very good. Others are afflicted by an ungentlemanly exulting in the power they supposedly wield over a restaurant's fortunes, the poor benighted saps."

Turner, who has since taken his Hungry Londoner website offline, says it was a "private email", and that he pays "for almost all" of his reviews.

However, he seemed to harm his own defence, by adding: "I've been approached by dozens and dozens of restaurants who have offered me free food and I've taken them up on it. And if it's no good, I sometimes don't think it's fair to necessarily write an article at all."

It's only fair to point out that Turner says his blogging is a "hobby". He adds: "I've been badly treated and misunderstood. At the end of the day, I'm just a guy who likes to write about my visits to restaurants."

According to chefs and foodie industry watchers, Turner is far from alone though, posing the question: what can restaurant-goers really trust online?

Be wary of sites that only post glowing reviews and check several sources before you dine, warns Stefan Chomka, the editor of Restaurant magazine. He says: "I speak to chefs all day long and most of them have stories of food blaggers, either people with blogs who approach restaurants in the hope of free meals, or others who use the threat of a bad Trip Advisor review to get another meal for free."

The food blogging world has got so big that there's even an industry conference cashing in on the boom. It's called Food Bloggers Connect and has been running since 2009. This year's event will be held in June in central London, with frustrated food writers paying up to £300 for a series of "skill-building panels" and sessions on how to "grow" their photography, writing and social media skills.

This isn't how food blogging started out, explains Chomka: "When food blogs first came onto the scene, the ideas was that it would be for the non-critic everyman to go a restaurant, pay the bill and then give a brutally honest opinion of what they've eaten. Since then, the industry and public-relations firms have seen the opportunity to court bloggers, so a lot of power they had has been diluted – and almost destroyed in some cases."

The recent Twitter publicity about Hungry Londoner – there are several other blogs with this name that are unconnected to Turner – was followed earlier this week by a rant from the foodie broadcaster and Guardian restaurant critic Jay Rayner.

The not unforthcoming critic took to Twitter to slam an "effing blogger who doesn't think that taking a freebie in any way degrades the validity of their opinion".

The "effing blogger" in question was Dominic Rowntree, who writes Samphire and Salsify, and the row erupted after the newspaper critic and the blogger both reviewed a new restaurant, Lanes of London, giving it widely differing verdicts. Rayner wasn't a fan; Rowntree was.

"My response is and always will be, that shit food is shit food and if something tastes good, then it tastes good," Rowntree tells The Independent. "Quite simple. Whether I was invited along for free or not can't change that".

On Twitter, Rayner said the blogger's defence was "bollocks" and that, effectively, he "was in the pay of the restaurant".

Rowntree, perhaps unsurprisingly, doesn't see it that way: "If there's a restaurant I particularly want to go to and I receive an email inviting me down to eat for free, I don't think it makes me quite as evil or ethic-less as Jay Rayner's making out, to accept the offer.

"As long as I make it clear that I didn't pay for the meal, which I do if I'm ever invited somewhere, then surely it's up to the reader to make their own mind up."

Yet if you delve deeper into this world, there are sinister suggestions floating around that some bloggers may have reportedly expected payment to attend events or post positive reviews. It's a strong allegation and, unsurprisingly, public-relations professionals in the restaurant industry were reluctant to talk to The Independent about it for this article. But it is common practice for restaurants to offer free meals to bloggers.

For restaurant goers, plenty of research, it seems, is the only way forward, according to Lewis at Gauthier Soho. "The average couple eating at Gauthier spends £120 to £150 a head, so before making that decision they'll have checked us out in four of five difference places, including newspaper reviews, a smattering of blogs, Google itself, which is now linked to the Zagat Guide, and on Twitter."

He adds: "Everything is intertwined and bloggers are part of it. It's all about trying to find the ones with integrity. Thankfully, if people are trying to abuse this area, they can't do it for very long because they will be exposed."

One blogger of whom professional critics and restaurateurs both speak highly is Chris Pople of the Cheese and Biscuits blog, who is known for sometimes cutting reviews. He works in market research, but started reviewing restaurants on his blog eight years ago when, he says, "there were no entirely positive reviewers out there".

"It took me a year and a half before anyone offered me anything for free," says the blogger, who estimates that he pays for around 75 per cent of his meals himself. "And I always say at the end of the post who paid for the meal. It wasn't about doing it to get anything for free, I was doing it to write about restaurants. The problem is now that you could start a food blog tomorrow and have your first free meal in a couple of hours. It's completely brazen."