Organic food is no healthier than conventional produce, shows a comprehensive review of 50 years of evidence.

Scientists at the London School of Hygiene and Tropical Medicine spent 12 months reviewing nutritional research on foodstuffs produced normally and under organic standards which ban artificial fertilisers and other chemicals.

Which? said the conclusion that there is no significant nutritional difference between the two systems could prompt shoppers to think twice about buying organic food. But organic farming experts questioned why the study dismissed evidence from some papers.

Funded by the Food Standards Agency, researchers searched scientific journals for all reviews of organic and non-organic fruit, vegetables, meat and dairy between 1958 and February 2008.

Of 162 relevant studies, they deemed 55 to be of "satisfactory quality".

They found no significant difference between organic and non-organic farming for 20 of 23 nutritional categories including vitamin C and iron.

Organic food had higher levels of phosphorous and acidity and conventional food was higher in nitrates. When all 162 papers were considered, organic farming was higher in 11 nutrients.

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

The FSA stressed that while it was neither for nor against organic food, it considered that the long-running health debate was over.

"This study does not mean we should not eat organic food," said Gill Fine, its director of dietary health.

"What it shows is that... there is no evidence of additional health benefits from eating organic food."

The findings accord with the FSA's previous advice and echo the views of government ministers that eating organic food is a "lifestyle" rather than a nutritional choice.

Peter Melchett, policy director of the organic certifier the Soil Association, said raw data showed some nutrients were far higher in organic food, such beta carotenes, 53 per cent higher and flavanoids, up 38 per cent.

He added: "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."

The independent research did not consider a £12m, four-year study by Carlo Leifert, professor of ecology at Newcastle University.

In a peer-reviewed paper last year (after the February 2008 cut-off date set by the FSA study), his EU-funded study found organic milk contained about 60 per cent more antioxidants and beneficial fatty acids than regular milk.

Results from his crop studies, not yet been published, suggest that organic wheat, tomatoes, cabbage, onions and lettuce have 10 to 20 per cent more vitamins.

Professor Leifert said: "With these literature reviews you can influence the outcome by the way you select the papers you use for your meta-analysis. 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 significant differences, and they come to the conclusion that there are no systematic nutritional differences."

In the past decade, shoppers have bought ever larger amounts of organic produce, with sales hitting £2.1bn last year, although growth has slowed markedly, and even started to reverse, during the recession. Nine in 10 households now buy organic food, but it still accounts for just 1 per cent of total food sales.

Organic farms rely on manure and other materials instead of "un-natural" chemicals, tend to be smaller and have higher standards of animal welfare.

In polling by TNS last year, shoppers rated "quality and taste" ahead of other factors such as the absence of pesticides and food additives as their main motivation for buying organic.

Sue Davies, of Which?, said: "Our research shows that people buy organic for a number of reasons, one of these being the perception that it's nutritionally better than conventional food. This research may make some people think twice before buying organic produce, but other people buy it for reasons such as pesticide or animal welfare concerns."

Britain's biggest organic farmer, Guy Watson, who has about 40,000 customers, described the research as "disappointing", adding: "In terms of the organic market industry as a whole, this is clearly not going to be helpful."

But he questioned whether householders would desert his own Riverford vegetables boxes because their primary motivation was not health.

"People buy from us really for flavour and freshness and second, because they trust us in a general way."

The National Farmers' Union said it had always maintained that conventional and organic food were equally healthy. "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," a spokesman said.

"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."

The Soil Association said organic farms were much richer in wildlife. "Consumers who purchase organic products are not just buying food which has not been covered in pesticides, they are supporting a system that has the highest welfare standards for animals, bans routine use of antibiotics and increases wildlife on farms."