It was decided that given the proven advantages it would have been unethical to continue a trial, in which half the women were given the new drug, letrozole, and half a placebo. The drug will now be offered to all the women involved.

The results are likely to influence the treatment of breast cancer for tens of thousands of women, significantly increasing their chances of long-term survival.

All 5,187 post-menopausal women in the trial, led by the National Cancer Institute of Canada and supported by the drug company Novartis, had previously undergone surgery to remove breast tumours, followed by five years’ treatment with tamoxifen.

Earlier trials have shown that prolonging tamoxifen treatment for longer than five years had no additional benefit. Letrozole belongs to a new class of drugs that operate by suppressing production of the female hormone oestrogen. Several other trials are in progress to see if other similar drugs are equally effective. It works by suppressing the production of oestrogen, which can influence the spread of cancer, by more than 95 per cent.

The size of the effect was surprising. After just under two and a half years, results showed that 207 women had suffered a recurrence of their cancer, of which 75 were in the group given letrozole and 132 in the group given a placebo — a 43 per cent reduction in the risk. The team responsible for the trial, led by Paul Goss of the Princess Margaret Hospital, Toronto, estimated that this meant an improvement in the four-year disease-free survival rate from 87 per cent to 93 per cent.

The New England Journal of Medicine concluded that in women who were past the menopause “letrozole therapy significantly improves disease-free survival”.

Professor Ian Smith, of the Royal Marsden Hospital in London, said: “This is one of the most important advances in the treatment of post-menopausal women with breast cancer.”

While breast cancer remains a major killer, there are signs of progress. The most recent figures show that almost three quarters of patients in England are alive five years after diagnosis — just less than the European average of 76.1 per cent.

Death rates for breast cancer have fallen by about a third since 1990 — the principal reason is the use of hormonal treatments such as tamoxifen.

Letrozole is already licensed for use in Britain at a cost of £83.16 for one month’s supply, ten times the price of tamoxifen. While the cost per patient is not huge, there are tens of thousands of women who could benefit from it so the total cost to the NHS could be large.

Without official approval, the drug is likely to be used less widely in Britain than in other countries, with the danger of “postcode” prescribing.

Professor Jack Cuzick, Director of Cancer Research UK’s Department of Epidemiology, Mathematics and Statistics, two weeks ago launched a trial of the AstraZeneca drug anastrozole (Arimidex) which seeks to test its protective effects on 10,000 women over ten years.

He said of today’s results: “They are exciting and point the way towards the future of breast cancer treatment.”

The only drawback identified by the trial is that letrozole has side-effects, including an increased risk of osteoporosis. In the group taking the drug, 5.8 per cent were diagnosed with osteoporosis against 4.5 per cent of those on placebo.

Because the trial was terminated early, the long-term impact of these side-effects has not been fully assessed, according to two US specialists in the New England Journal of Medicine.

They accept that the organisers of the trial had no alternative but regret that the opportunity to assess fully the risks and benefits of the drug over five years had been lost.

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