Danish researchers have found that exposure to organochlorines increases a woman's chances of developing the deadly disease.

They believe the pesticide mimics the female sex hormone oestrogen.

Women who never have children or who experience a late menopause are known to be at a higher risk of developing breast cancer.

This is thought to be because their production of oestrogen remains uninterupted for a long period of their adult lives.

The Danish researchers scrutinised blood samples taken in 1976 from 7,712 women to test the link between organochlorines and breast cancer.

In 1996-97 they tested new blood samples for the presence of 48 substances from 268 women who had developed breast cancer.

'Significant' link

Writing in the Lancet medical journal, they claim one organochlorine pesticide known as dieldrin was "associated with a significantly dose-related risk of breast cancer".

Dr Annette Pernille Hoyer, who worked on the study, said: "I fear that the link is significant.

"The use of pesticides should be reduced as much as possible in general.

"Human beings are naive when they do not believe that a poison designed to kill living organisms does not harm them.

"We should have as healthy food and a clean environment as possible."

Dr Pernille Hoyer said a test was needed to measure the disruption that substances can cause to the hormonal system.

Doubts raised

Professor David Phillips, of the Institute of Cancer Research, questioned the value of the research.

He said other studies had cast doubt on the theory that organochlorines acted as weak oestrogens.

"This research tested 48 compounds and only one showed any significant association with breast cancer - that is not far outside the bounds of coincidence," Professor Phillips said.

DDT: An organochlorine pesticide which has been banned

"This study is part of the continuing effort to understand why breast cancer has increased in recent years, but is not likely to provide a major reason."

Professor Phillips added that most organochlorines were being phased out in the UK.

Other organochlorine pesticides include Lindane, atrazine and DDT.

Atrazine is the most common pesticide found in UK drinking water, whilst DDT has been banned in the developed world for many years.

DDT metabolites have recently been shown to block the action of male hormones.

Workers exposed to high levels of the pesticide chlordecone suffer decreased sperm motility and abnormal sperm.