Women who always or mostly eat organic foods are no less likely to develop cancer than women who eat a more conventional diet, according to a study published today.

Using data from the Million Women Study, the biggest health research project in the UK, Cancer Research UK scientists from Oxford University found that eating an organic diet grown without pesticides made no difference to overall cancer risk.

The authors of the paper, published in the British Journal of Cancer, said that the results were "particularly relevant given that health concerns have been identified as the primary motivation for consumers' purchase of organic food". Professor Tim Key said: "The overall conclusion is really simple – we don't see any difference in the total risk of any type of cancer, depending on whether people said they choose organic food. It's a very large study so the overall result is very robust."

Past studies have suggested, albeit inconclusively, that pesticides could increase the risk of risk of cancer. Pesticides are widely used in agriculture, although there are limits to the amount of residue that can be present in food, and the Oxford researchers described the quantities present as low.

They asked around 623,080 women aged 50 or over about whether they ate organic foods, and then tracked the development of 16 of the most common types of cancer over a nine-year period.

They found no difference in overall cancer risk when comparing the 180,000 women who reported never eating organic ally grown food with around 45,000 women who said they usually or always eat organic food.

However, differences did emerge with respect to breast cancer and non-Hodgkin lymphoma when they looked at the results for the 16 types of cancer individually.

The researchers found a small increase in risk for breast cancer but a 21% reduction in the risk for non-Hodgkin lymphoma in women who mostly ate organic food. Of the 180,000 women who never eat organic food, 685 developed that cancer while there were 133 cases among the 45,000 women who said they usually or always eat organic products.

As the analysis of individual cancers involved smaller numbers, Key said that the results were less robust and could be down to "chance" or other factors. But he said that, given previous studies had suggested a link between pesticides and non-Hodgkin lymphoma, it would be worth investigating more closely.

Peter Melchett, director of policy at the Soil Association, which promotesthe trade body for organic food and farming, launched a wide-ranging attack on the study. He said it was strange that the 21% decrease in non-Hodgkin lymphoma among women who reported usually or always eating organic food was being "so readily dismissed" by Cancer Research UK, and said the reasons for buying organic foods varied, and ranged from the benefits to wildlife to the fact that organic standards prohibit GM crops and ingredients, harmful hydrogenated fats and controversial artificial food colours and additives. "People also buy organic to reduce their exposure to pesticides – 320 of which can be routinely used in non-organic farming." He also claimed that the study had a number of weaknesses compared with other research, including that women's BMI and the amount of physical activity they took part in were only measured once during the study.