However, scientists have urged caution, as the drug has also been linked with more than a doubling in the risk of blood-clotting complications especially after surgery or long periods of immobility.

They say women who take the drug might be wise to take low-dose aspirin before long flights or major surgery to minimise the risk of blood clots.

It is vital for women at high risk of breast cancer that we establish firmly the benefits of taking tamoxifen for preventing the disease versus the potential risks

Sir Paul Nurse

Tamoxifen is already widely prescribed for women who have been diagnosed or treated for breast cancer, and is widely accepted as drastically reducing the rate of recurrence.

But evidence is mounting that it is also effective at preventing the disease in the first place.

Professor Jack Cuzick from Cancer Research UK, who led the new study, said: "All along the line we have kept the volunteers in the trial fully informed of developments.

"That is why we felt it was right to report the preliminary findings ahead of publication."

He said the possible side effect was explained to all women before they joined the trial and they were recommended to take a break from medication prior to certain operations.

Balance of risks

Professor Cuzick said it was important that women with breast cancer who are taking tamoxifen continue to do so because the benefits far outweighed the potential side effects.

The International Breast Cancer Intervention Study involved around 7,000 women from the UK, Australia, New Zealand and Europe.

The women were aged between 35 and 70 years and were at an increased risk of breast cancer because they had a family history of the disease or had a benign lesion associated with an increased risk.

The study found the frequency of breast cancer was reduced by more than 30% among women given the drug.

Sir Paul Nurse, Chief Executive of Cancer Research UK, said: "Cancer Research UK will continue to fund the IBIS trial to its conclusion.

"It is vital for women at high risk of breast cancer that we establish firmly the benefits of taking tamoxifen for preventing the disease versus the potential risks."

Tamoxifen works by neutralising the action of the female hormone oestrogen which stimulates breast tumour growth.

The research is published in The Lancet medical journal.