HRT may be able to reduce the risk that women with heart disease go on to develop diabetes as well.

However, there is still no firm recommendation that women with heart problems should take it.

HRT is already thought to slightly increase the risks of both heart disease and breast cancer.

The results of the "HERS" study led by the University of California at San Francisco were published in the Annals of Internal Medicine.

They looked at more than 2,700 postmenopausal women suffering from heart disease.

The potential benefit to patients for one health outcome needs to be weighed against the risks Dr Alka Kanaya, UCSF

Some already had diabetes, but others did not, or had "impaired fasting glucose" - a condition with some similar traits to diabetes but not severe enough to be called diabetes.

The women were given either oestrogen and progestin HRT, or a dummy pill, and followed up for four years.

Over that period, in the dummy pill group, there were 98 new diagnoses of diabetes.

In the HRT group, there were only 62.

This increased risk, say the researchers, could not be explained by other factors separating the two groups.

Sugar control

In addition, the dummy pill group had significantly greater worsening of their ability to control their blood sugar over the four years compared to the HRT group.

However, the findings are unlikely to encourage many doctors to prescribe HRT with this benefit in mind.

Recent large studies have cast a shadow over the long-term safety of HRT.

Dr Alka Kanaya, who led the research, said: "The potential benefit to patients for one health outcome needs to be weighed against the risks for others, such as coronary events and breast cancer."

Penny Williams, a care advisor for the charity Diabetes UK, said: "These are interesting results for women who already need HRT but we wouldn't recommend it's use to prevent diabetes.

"If women want to reduce their risk of developing diabetes they should try to eat healthily and take regular exercise.

"More research is needed in this area and we look forward to seeing any results."