Identifying and monitoring these susceptible women is going to be an important challenge

Professor Julian Peto

"We now think that many, possibly the majority, of breast cancers occur in a minority of women with an inherited risk.

"Identifying and monitoring these susceptible women is going to be an important challenge."

Cancer risk

Professor Peto, who works at the Institute of Cancer Research and the London School of Hygiene and Tropical Medicine, studied data about 1,300 pairs of identical twins and 1,000 pairs of non-identical twins.

He found that at least one-third of the identical twins went on to develop breast cancer, compared to just one in nine other women.

Non-identical twins of breast cancer patients were found to have a much smaller risk.

He said this suggested it was the shared genes that caused the problems rather than the fact that the twins had shared a womb or been brought up together.

Professor Peto also found that if one twin developed cancer at an early age then the other was likely to get the disease within 20 years.

Research

Sir Paul Nurse, of Cancer Research UK said: "Knowing that the identical twins of women with breast cancer have such a high risk is of practical use, since we can carefully monitor and counsel these women more effectively.

"The research will also play a vital role in clarifying our picture of how the disease develops and particularly how a woman's genes can combine to increase her risk of the disease."