Children who eat an apple or pear a day may be exceeding the pesticide safety threshold because of residues on the fruit, according to research.

Using Department of Environment data on pesticides on fruit collected from supermarkets, scientists calculated that each day some children would get a toxic level of pesticides.

The research, to be published on Sunday, says the government repeatedly claims that the levels of pesticide are safe because, instead of measuring individual apples, researchers buy 10, mash them and take an average reading to see if they are safe. This is the internationally agreed method of checking residues.

But government figures show that the pesticide is not evenly spread across the batch, and one or two apples could contain 90% or more of the pesticide in the batch.

The research, published in the International Journal of Occupational and Environmental Health, is from Andrew Watterson of Stirling University, and Vyvyan Howard of Liverpool University. It used mathematical modelling to measure exposure to pesticides for children aged between 18 months and four years old. The pesticides involved can disrupt children's hormones and some are suspected of causing cancer.

The study found that between 10 and 220 young children a day could be exposed to pesticide residues at levels which could pose immediate and long term threats to health.

The good news for British fruit growers is that samples grown in this country had lower residues than imported fruit, so buying home-produced produce fruit will reduce the danger, said Emily Diamand, Friends of the Earth's senior food researcher and one of the authors of the report.

Ian Brown, chairman of the government's pesticides residue committee and a consultant toxicologist at Southampton University, said he was confident the maximum residue levels found in fruit tested would not harm a toddler. The safe level for humans was set 100 times lower than a level which had no observable effect on rats.