British scientists say there is no question that their trials into the effects of the anti-cancer drug Tamoxifen will be halted as a result of the encouraging results released by American scientists on Monday.

Scientists from the US National Cancer Institute said that their 6-year study of Tamoxifen showed that it reduced the risk of breast cancer by 45% in women who were at risk of the disease.

Women involved in the American study who were taking dummy drugs have been informed that they can start taking Tamoxifen if they wish.

However, British doctors leading another major Tamoxifen study have criticised the Americans for releasing the data too quickly, potentially sabotaging the British research.

There are still major questions left unanswered say the British researchers, such as whether Tamoxifen can reduce deaths from cancer as well as the incidence of the disease.

Both the American and the British trials were aimed at women at a high risk of breast cancer because of their family history and their age.

Delyth Morgan, Chief Executive of Breakthrough Breast Cancer, said "The results are undoubtedly important and exciting, but point up the need for urgent long-term research."

Dr Trevor Powles of London's Royal Marsden Hospital, who has been researching Tamoxifen for over ten years, said "The Americans have unblinded their trial which means it will be unbalanced and they will not be able to answer many questions."

American researchers have defended their actions saying that when it was shown that Tamoxifen had such a dramatic effect it would have been unethical to continue giving women at risk dummy-drugs.

They say that the trials primary aim, to demonstrate whether Tamoxifen was effective in preventing breast cancer, has been achieved.