GPs have been warned that prescribing a certain regimen of hormone replacement drugs can double the chances of breast cancer.

The government's committee of medicine safety has written to GPs to alert them to the risks following the study.

The deputy chief medical officer has today called for calm following reports.

Whilst the committee stops short of advising doctors to stop prescribing HRT drugs it says they should discuss the risks and benefits of the treatment.

The Million Women Study revealed that post menopausal patients who are prescribed the combined oestrogen-progestagen treatment run a greater risk of developing breast cancer than those using other types of HRT.

The committee advises that for short-term use of HRT for the relief of menopausal symptoms, the benefits outweigh the risks for many women.

However, for longer-term use of HRT, women should be aware of the increased risk of breast cancer and other adverse effects.

The committee advised that each decision to start HRT should be made on an individual basis.

It says that treatment should be regularly reappraised - at least on an annual basis.

Committee chairman Professor Gordon Duff said: "We have reviewed the study very carefully and realise that these findings may present women and their doctors with a dilemma about long-term HRT treatment.

"However, what this study clearly shows is that, in all cases, the additional risk of breast cancer begins to decline when HRT is stopped and by five years reaches the same level as in women who have never taken HRT.

Professor Duff said the new findings mean "it is even more important" that the decision to start HRT is made on an individual basis.

"Women who are concerned by these new findings should make a routine appointment to discuss these matters with their doctor," he added.