It comes a week after a US study showed a pill called Prempro may increase the risk of breast cancer, stroke and blood clots.

Authorities ordered researchers to stop the trial three years early.

Around 5,000 women in the UK are taking the drug as part of a separate study.

Two committees of the Medical Research Council are meeting this week to discuss whether or not to halt the British trial.

A spokesperson told BBC News Online that a decision was expected on Friday.

'Small' risk

The new warning is disclosed in the same journal that is publishing the results of the first study, the Journal of the American Medical Association (JAMA).

The latest trial involved more than 44,000 US postmenopausal women who took oestrogen-only HRT for up to 20 years.

It found that women who took the treatment for 10 years or more were at higher risk of developing ovarian cancer.

In this country women generally take combined HRT and the study found no increase in risk for that group

Dr Anne Szarewski, Cancer Research UK

But experts say that British women should not be alarmed.

Peter Bowen-Simpkins, spokesperson for the Royal College of Obstetricians and Gynaecologists said the risk of ovarian cancer was "very, very small".

Professor David Purdie of the centre for metabolic disease at Hull Royal Infirmary said very few women in the UK are prescribed oestrogen-only HRT and for much shorter lengths of time than in the US.

"Very few women take oestrogen for 20 years in this country," he told BBC News Online.

Limited relevance

Dr Anne Szarewski, clinical consultant for the charity Cancer Research UK, said women in the UK should not be alarmed as it had limited relevance to them.

"In this country women generally take combined HRT and the study found no increase in risk for that group," she said.

Oestrogen-only HRT is widely prescribed to women who have had a hysterectomy.

British women of menopausal age who have a hysterectomy are generally advised to have their ovaries removed at the same time, said Dr Szarewski.

This avoids a future risk of ovarian cancer.

In the US study, some women had undergone hysterectomy but all had at least one ovary.