Breast cancer tumours in women using hormone replacement therapy (HRT) are no less dangerous than those in non-users, claim researchers.

The finding contradicts previous thinking which suggested an improved cancer prognosis for HRT users.

Now doctors at Glasgow Hospitals University NHS Trust say there is no evidence of any difference in type, size or grade of tumours.

They looked at 1,130 women aged between 50 and 64 who were routinely screened by the West of Scotland Breast Screening Unit between 1988 and 1993 and in whom a cancer was detected or developed between screenings.

The women were asked if they were using HRT at the time of screening and those who were found to have a cancer were asked again at the time of assessment.

There were 815 women with screen detected cancers - 100 of those were using HRT when they were screened. A total of 66 of the 298 women who developed cancer between screenings were HRT users.

Tumour size

Of the women using HRT, 24% developed well differentiated tumours, compared to 22% of non-users. There was no difference in average tumour size.

Sheila Stallard, a staff grade surgeon in Glasgow, says in a report published in the British Medical Journal (BMJ): "Our results do not support the commonly held view that women using hormone replacement therapy develop tumours with favourable prognostic features."

But she added that neither did they have a worse prognosis, which she said was "reassuring" to doctors prescribing HRT.

The report does, however, confirm the belief that women who use HRT are more likely to have a cancer missed during screening.

Those who developed the disease within a year of screening were twice as likely to have been using hormone replacements.

A separate study, published on Friday in the Lancet medical journal, concludes that the long-term prognosis of women with very small breast cancers can be reliably determined using mammograms.

Mr Nigel Sacks, a consultant breast specialist at the Royal Marsden Hospital, London, said he was surprised by the BMJ study's finding on the tumours' prognosis.

"Most studies previously have shown that people diagnosed with breast cancer who are on HRT have a better cure rate," he said.

But he added: "There is a big difference between being diagnosed with breast cancer and dying of it."

A spokesman for Breast Cancer Care said: "This report confirms that there is no evidence to suggest that taking HRT will have any effect, favourable or adverse, on prognosis for women developing breast cancer.

"Nevertheless, the risk of getting breast cancer, although small, is still there and women considering taking HRT need to weigh up the positive benefits and possible side effects."

One third of menopausal women in the UK take HRT, and it is known to marginally increase the chances of developing breast cancer by around 2.3%.

More than 1,000 women die every month from the disease and it is the biggest killer of women under 50 in the UK.