Researchers say there is now firm evidence that organically-grown produce is healthier to eat than conventional crops.

The Soil Association, the group which campaigns for organic farming, has told BBC Radio 4's Costing the Earth programme that organic crops contain more nutrients.

Director Patrick Holden said research has shown that they contain more secondary metabolites than conventionally-grown plants.

Secondary metabolites are substances which form part of plants' immune systems, and which also help to fight cancer in humans.

Mr Holden said organic crops also have a measurably higher level of vitamins, and that this can benefit people who eat them.

By contrast, he said, "intensive farming is devitalising our food".

Mr Holden said the research, from Denmark and Germany, would be presented in the UK at the association's conference on organic food on 8 January.

'Dangerous delusion'

The researchers' findings will strengthen the organic lobby, which has been accused of making exaggerated and even unwarranted claims.

The programme spoke to scientists who said they knew of no evidence of any nutritional benefit from eating organic food.

Others claimed it could be positively dangerous, especially when it was fertilised with sewage containing potentially harmful organisms.

Some maintained that many of the natural pesticides produced by plants were potentially more of a risk than the synthetic ones used in conventional agriculture.

And with organic food costing appreciably more than ordinary products, one US cancer specialist said organic farming was a "dangerous delusion".

Poor people would find it hard to afford the fruit and vegetables they needed to reduce their cancer risk, he argued.

But the World Health Organisation has estimated that between 3.5 and 5m people globally suffer acute pesticide poisoning every year.

Organic food, the programme concludes, has individual advantages and disadvantages but overall it is almost certainly beneficial on the broader scale.