Screening is more likely to miss breast cancer if the woman has been taking hormone replacement therapy, according to a study.

And the overall decrease in accuracy means that women taking HRT are also more likely to be given a false positive scare.

It is thought the errors creep in because HRT could be making breast tissue more dense and harder to scan.

The research, from a team at La Trobe University in Melbourne, Australia, published in the Lancet medical journal, looked at the cases of more than 100,000 women undergoing mammography.

The screening sensitivity was only 64.8% in the HRT group, compared with 77.3% in non-users.

In the group of 50 to 69-year-old women usually targeted by screening programmes, sensitivity was 64.3% in the HRT group and 79.8% in non-users.

False negative results - those which miss an existing cancer - were 60% more likely in HRT women.

Unnecessary operations

False positives, in which the woman is told she probably has cancer when in fact she does not - usually leading to an unnecessary biopsy operation, were 12% more likely among the women taken HRT.

Dr Anne Kavanagh, from La Trobe University, said: "An additional 23 cancers, or 20% more cancers, would have been detected by screening among HRT users if the sensitivity of mammography was the same."

The team has called for their results to be verified by other researchers.

If this happens then individual HRT preparations could be tested to find the one which has the smallest effect, or women could be advised to discontinue HRT for the weeks leading to a mammogram.

HRT is thought to confer health benefits to woman, particularly after the menopause, helping prevent hip fractures by maintaining bone density.

The breast screening programme in England is set to be extended to cover women between 65 and 69, despite claims from experts that it does not actually save lives.