Industrial pollutants and agricultural pesticides do not cause breast cancer, a survey shows.

In the largest study to date, researchers at the Yale Cancer Centre did not find a significant relationship between exposure to polychlorinated biphenyls (PCBs) or the pesticide DDE and the risk of breast cancer in women.

They studied 1,000 women in Connecticut.

Blood serum levels of PCBs and DDE were analysed for an association with breast cancer risk.

No significant difference in serum levels was found between the women with cancer and the 1,000-strong control group.

The question whether environmental exposure to organochloride compounds increases the risk of female breast cancer has been the subject of debate in recent years.

However studies relating to PCBs and DDE have always proved inconclusive.

Lead author of the study, Professor Tongzhang Zheng, said: "I am hopeful that this, our most recent and conclusive study, will put to rest the notion that exposure to DDE causes breast cancer.

"Based on these results, it is also unlikely that total PCBs have a major impact on the risk of breast cancer."

In 1998, Danish researchers found evidence that exposure to similar pesticides called organochlorines, such as dieldrin, was associated with a significantly dose-related risk of breast cancer.

They believed the pesticide mimicked the female sex hormone oestrogen.

The value of previous research has been questioned by Professor David Phillips, of the Institute of Cancer Research, who has said other studies cast doubt on the theory that organochlorines act as weak oestrogens.

Most organochlorines are being phased out in the UK because many have been linked to medical problems.

DDT, which has been banned in the developing world for many years, has recently been shown to block the action of male hormones.

And workers exposed to the pesticide chlordecone have been shown to suffer decreased sperm mobility and abnormal sperm.