A MAJOR question mark was hanging over hormone replacement therapy last night after a massive study confirmed a definite link with breast cancer.

In the case of combined oestrogen-progestogen HRT treatment, the study of more than a million UK women showed that the risk actually doubled.

These dramatic findings mean that hundreds of thousands of women now face a dilemma over whether to stop taking pills which help them overcome menopausal symptoms.

Researchers estimated that the use of HRT by women aged 50 to 64 in the UK had resulted in an extra 20,000 breast cancers in the past decade, with combined HRT accounting for 15,000 of these cases.

But experts say that women should discuss the risks and benefits of treatment with their GP rather than stop immediately.

Breast cancer survivor Mary Lee, 60, from Stokesley, North Yorkshire, said: "This is going to frighten a lot of women because breast cancer is every woman's fear."

But Mrs Lee urged women not to take panic measures.

"I know people who are on HRT who really benefit from it," she said. "You need to talk it over when you next see your GP."

The research showed that those who used the combined treatment were twice as likely to develop breast cancer as those who had never been on HRT.

But even women who take treatments containing a single active ingredient were at increased risk.

The risk increased by 45 per cent among users of treatment containing only tibole, and by 30 per cent among users of oestrogen-only HRT.

The study also shows that if a woman stopped taking HRT, within a few years her risk of developing breast cancer dropped back to that of non-users.

The Committee on Safety of Medicines has now written to all health professionals, urging doctors to carefully discuss with their patients the risks and benefits of treatment and review each case individually.

They said the results of the study did not mean any urgent changes to women's treatment were necessary and for most women the benefits of having HRT would outweigh the risks.

Cancer Research UK's Epidemiology Unit in Oxford monitored more than one million women between the ages of 50 and 64 who joined the study between 1996 and 2001. Half the women were using HRT or had done so in the past.

After 2.6 years of follow-up, 9,364 cases of breast cancer were found. After 4.1 years, 637 breast cancer deaths were registered among all the women studied.

Past studies have shown a link between HRT and breast cancer risk, but this is the first and largest project to discuss the increased risks relating to different types of the treatment.

Dr John Toy, medical director of Cancer Research UK, said: ""It would be sensible for a woman to take HRT for only as long as it is necessary to deal with her medical problems as advised by her doctor.

"A woman wanting to take HRT for a long time would be extremely wise first to consider carefully the findings of this large study and other relevant research."

Professor Reg Hall, lead clinician of the Northern Cancer Network, said: "This study means there will be a serious rethink of HRT. It is an extremely important study which reminds us that every woman should think very carefully before starting this treatment."

Prof Hall, who is based at the Freeman Hospital in Newcastle, added: "Women need to be fully informed about the pros and cons of HRT. Some of the downside of the menopause, such as an increased risk of osteoporosis, can be prevented by taking more exercise."

Dr Andrew Oakenfull, Ferryhill GP and chairman of County Durham Local Medical Committee, agreed that family doctors are sometimes pressured to prescribe HRT. "Patients look at Tina Turner and say I wish I could look like her - put me on HRT," he said.

Dr Oakenfull said women already on HRT should wait until their next scheduled appointment with their GP before deciding whether to continue with treatment.

"There are risks and benefits which have to be balanced," he said. "Some patients say that life is unbearable without HRT."