Teenage girls who live or work on farms may be putting themselves at risk of developing breast cancer in later life, researchers say.

Pesticides or other toxic agents to which farm workers are exposed may be responsible for triggering changes in the developing breast, they said.

A study of 1,100 women, of whom half had been diagnosed with breast cancer, found those with the disease were almost three times more likely to have been farm workers, many during adolescence.

The researchers from the University of Stirling, who led the study conducted in Canada, said developing breast tissue was especially vulnerable to toxic exposures during adolescence.

But specialists in the epidemiology of cancer were sceptical that chemicals in the environment could increase the risk of breast cancer. They said there was still no good evidence for the link.

Breast cancer risk was highest among women who went on to get jobs in the car industry and in health care, after working on farms, according to the study, published in Annals of the New York Academy of Sciences. The researchers say the kind of jobs a woman has, and the substances she is exposed to, may affect her chances of getting breast cancer and should be more extensively studied.

Professor Andrew Watterson, a researcher on the paper, said the findings also had implications for the UK. More research was needed on the link between professions and cancer, he said, adding: "This is a really big public health issue."

Estimates suggest about 4 per cent of all cancers are related to or caused by occupation, he said. He cited East Anglia as an area where large numbers of young women were working on or growing up on farms.

The authors said there was evidence of an association between breast cancer and some pesticides. Although working in healthcare or the car industry did not significantly raise the risk, it was raised in women who had worked in farming.

They added: "Nurses and other healthcare workers are potentially exposed to ionising radiation... drugs, anaesthetic waste gases, and viruses possibly associated with cancer risk. A number of hormonally active chemicals are, or have been, used in medicine and laboratory work." A range of solvents used in car manufacturing may also be linked to an increased breast cancer risk, they said.

They concluded: "While occupational categories in this study serve as surrogates for exposure, it is plausible that exposure to agricultural chemicals is a causative factor. Because many women who worked in farming began during adolescence, it is plausible the timing of exposure is of significance in terms of risk."

Henry Scowcroft, the science information officer for Cancer Research UK, said: "This study only looked at a relatively small number of women. This is too small a group to be able to draw any firm conclusions about a possible link. Nor does it say anything at all about what might be behind the observed increase in risk for some occupations.

"Breast cancer is on the rise in Western society. All the available scientific evidence shows this increase is largely down to changes in lifestyle factors.

"Scientists have also looked at whether environmental toxins might also contribute to the increase in breast cancer, but have found no conclusive link."