A senior adviser to a medicines watchdog has resigned because he disagrees with a health warning issued on hormone replacement therapy.
HRT has been linked to cancer
The UK Committee on the Safety of Medicines says that women over 50 should look for alternative treatments to prevent osteoporosis.
Studies point to an increase in breast cancer risk among long-term users.
But bone expert Professor David Purdie has quit his role - claiming HRT could still be given to some over-50s.
HRT is known to be an effective way of preventing bone loss which can lead to osteoporosis in later life, and many women had been given the treatment for this reason, rather than to alleviate menopausal symptoms.
However, a study in August suggested that a small number of women taking HRT long-term would develop breast cancer as a result.
Professor Purdie, from the Edinburgh Osteoporosis Centre, is a member of the expert advisory group helping drawing up guidance to GPs in the wake of the study's publication.
On Thursday, the advice was issued, suggesting that alternative treatments to prevent osteoporosis should be used by women aged over 50, with HRT prescribed only where these had failed.
However, Professor Purdie told the BBC that GPs should still be able to give HRT to some older women to prevent osteoporosis.
He said: "I'm afraid I don't agree with it. [the guidance]
"I think that HRT has a major role to play in the prevention of bone loss, and the prevention of osteoporosis, particularly in younger women in their fifties, just after the menopause.
"The use of oestrogen is one of the front line treatments, has been for a long time, It is proved and tested."
He said that the "jury was still out" on HRT, with other research producing contradictory results about the likely threat to a woman's health.
Prior to news of the link to breast cancer, an estimated 1.5 million women in the UK were taking HRT, although a substantial number have stopped even in the past few months.
Professor Purdie said: "HRT still has a role to play in preventing bone loss and fractures.
"Women need to be told what the absolute increase in risk is for them - this is what matters, rather than the relative increase in risk for all women.
"There is, in my view, an association with breast cancer in the long term - but it is a small association, two to three cases per thousand."
A spokesman for the Department of Health confirmed that Professor Purdie had tendered his resignation from the group advising the CSM.
She said: "He should have said any concerns he had that were about the decision to the committee prior to the publication of the advice.
"All the evidence points to the decision the committee has come to on HRT - there are no plans to withdraw the advice."
Fellow committee member Dr Rosemary Leonard said that the agency was attempting to issue general guidance to GPs.
The decision whether to stop or continue with HRT was an individual one, she said, to be made in consultation with a GP.
"It's worth remembering that one in three women will get osteoporosis at some time in their lives, but there are alternative treatments, such as biphosphonates, which do not carry this additional risk of breast cancer."
The advice does not rule out HRT use for either the under 50s - whose breast cancer risk is much lower - or for those with menopausal symptoms, although they are advised to mimimise the amount of time they take it, and take the lowest possible dose.
Women already using HRT to treat osteoporosis are advised to have their treatment reviewed at their next appointment.
Dr Hamish Meldrum, from the GPs committee at the British Medical Association, urged patients not to panic, and said that it was still appropriate to use HRT to ease severe menopausal symptoms.
"There are other quite effective treatments for preventing brittle bone disease - in most cases they are as effective as HRT.
"If they are not proving successful, or if women cannot take them because of side effects, you could consider HRT as a second-line treatment."