Enough is known about the causes of breast cancer to make a vaccine or prophylactic drug a real possibility, a leading cancer expert said on Sunday.

Professor Valerie Beral of Oxford University, who leads the Million Women's Study into the causes of the disease, said the study had put beyond doubt what had long been guessed, which many breast cancers are caused by the absence of hormonal changes connected with childbirth.

Beral challenged the scientific community to turn its efforts to preventing breast cancer. While money and effort is poured into better drug treatments, hardly anyone is working on prevention.