Organic food is no healthier than conventional food, according to the world's biggest research project into the issue.

A review of scientific studies for the past 50 years found there were no significant nutritional differences between conventional produce and organic fruit, vegetables, meat and milk.

The findings, intended to answer once and for all a long-running controversy, attack one of the pillars underpinning organic food produced without artificial fertilisers and with higher animal welfare standards: that is is healthier for individuals as well as better for the environment.

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However organic farming experts criticised the study carried out by the London School of Hygiene and Tropical Medicine and questioned why it dismissed evidence it gathered that organic food is higher in some nutrients.

The Food Standards Agency commissioned the research to discover whether Britain's £2bn organic industry could claim higher health benefits for its products.

Of 162 scientific papers, researchers found 55 that were high quality and checked them for different minerals and vitamins such as Vitamin C and iron. In "satisfactory quality studies" there was no difference between the organic and non-organic farming in 20 of 23 nutritional categories. Organic was better satisfactory studies, organic food had "statistically higher levels" of phospohorous and acidity, but conventional was higher in nitrates.

Dr Alan Dangour, who led the study, said: "A small number of differences in nutrient content were found to exist between organically and conventionally-produced crops and livestock, but these are unlikely to be of any public health relevance.

"We found broadly that there was no important difference between organic and conventional produce."

The Food Standards Agency stressed that while people bought organic produce for several reasons, it considered that the long-running debate about whether they were more healthy was now over.

Gill Fine, FSA director of dietary health, said: "This study does not mean we should not eat organic food. What it shows is that there is little, if any, nutritional difference between organic and conventionally-produced food and that there is no evidence of additional health benefits from eating organic food."

The result accords with the FSA's previous advice that there was "no significant" health benefit from eating organic produce and also echoes the views expressed by previous Government Defra ministers that the case for organic food being healthier is unproven.

Supporters of organic farming, however, claimed that the results were flawed because of the criteria used to select the most important research.

When all 162 studies rather than the 55 highest quality ones were taken into account, organic farming was frequently higher in nutrients than conventional produce. For instance, beta carotenes were 53 per cent higher and flavanoids 38 per cent higher in organic food than non-organic food.

Peter Melchett, Policy Director at the Soil Association, complained: "The review rejected almost all of the existing studies of comparisons between organic and non-organic nutritional differences. This was because these studies did not meet particular criteria fixed by the London School of Hygiene and Tropical Medicine.

"Although the researchers say that the differences between organic and non-organic food are not 'important', due to the relatively few studies, they report in their analysis that there are higher levels of beneficial nutrients in organic compared to non-organic foods."

A leading academic, Carlo Leifert, professor of ecology at Newcastle University, also attacked the study. Professor Leifert has been conducting a £12m four-year EU-funded study. Some initial research, published last year in the Journal of Science, Agricultural and Food last year, found that organic milk contained around 60 per cent more antioxidants and beneficial fatty acids than normal milk.

Provisional results from another part of the same study, which has not yet been published, suggests that organic wheat, tomatoes, cabbage, onions and lettuce also had between 10 and 20 per cent more vitamins. Neither have been included in the FSA review.

"With these literarure reviews you can influence the outcome by the way that you select the papers that you use for your meta-analysis," Professor Leifert said.

"My feeling - and quite a lot of people think this - is that this is probably the study that delivers what the FSA wanted as an outcome. If you look at the differences they found – a 50 per cent increase in beta carotenes and a 30 per cent increase in flavanoids – they are quite significant differences, and they come to the conclusion that there's no systematic nutritional differences. That's just not very convincing."

During the past decade, sales of organic produce have soared, rising 22 per cent between 2005 and 2007, hitting £2.1bn last year.

Nine out of 10 households buy organic food. Britain has the third biggest market for organic food, after Germany and Italy, though it still only accounts for only 1 per cent of retail sales and 3.5 per cent of British farmland.

Although organic supporters have mostly trumpeted the environment as the biggest reason for buying organic, "quality and taste" is the biggest factor by buyers, shows research.

Britain's biggest organic farmer, Guy Watson, whose Riverford organic food network serves 40,000 customers, said the research was likely to hit his sales nationally. "In terms of the organic market industry as a whole, this is clearly not going to be helpful," said, adding that he did not believe his veg box scheme would be affected. "People buy from us really for flavour and freshness and secondly because they trust us in a general way."

The National Farmers Union said it had always said that both conventional and organic food to be "equally healthy and nutritious." The NFU said: "It is down to the consumer to choose what kind of food they wish to buy just as it is down to the individual farmer to decide which system he wants to employ – previous research has found there is no evidence to prove organic food is healthier than conventional and we believe there is space in the market for all models of farming to thrive and prosper."