The risk for women who have been taking HRT for more than three years had almost four times the risk of developing gallstones as women who did not take the therapy.

Women who had been taking HRT for less than three years had a 2.6-fold increased risk.

The researchers from the University of East Anglia, who presented their findings to the British Society of Gastroenterology, say there are many benefits to women taking HRT, but doctors must explain the gallstone risk.

Gallstones, solid pieces of material that form in the gallbladder, are most common in middle-age and can cause severe pain, vomiting and even jaundice.

Women develop gallstones, which can be as large as a plum, more commonly than men and at a younger age.

Reducing risk

The University of East Anglia looked at data for 13,433 women aged 45 to 79, who were enrolled in the European Prospective Investigation Into Cancer (EPIC).

Whether or not the women were taking HRT, and if they were, how long they had been taking it, was recorded.

The women were also followed up to see if they developed gallstones.

Fifty-eight had done so by March 2000. Further data, which has not yet been analysed, has identified around 40 more cases since that date.

Experts are not sure why the link between HRT and gallstones exists.

Dr Andrew Hart, senior lecturer in gastroenterology at the University of East Anglia, who led the research, said women who were considering taking HRT could be advised by their GP to take other measures which should reduce their gallstone risk.

These include keeping alcohol intake to a moderate level and maintaining a healthy weight.

He told BBC News Online: "I think this is a finding which GPs need to be aware of, and explain the likely risk to their patients."

But he added: "It's very important that this is put in context. HRT is a good treatment for post-menopausal symptoms and has a protective effect against osteoporosis and heart disease."