Writing in the British Medical Journal, public health expert Dr Jane Wells claims the debate has "assumed an importance out of proportion to its potential impact on public health".

Dr Wells, a specialist registrar in public health medicine at the Institute of Health Sciences, Oxford University, said trials of the effectiveness of screening for breast cancer found it significantly reduced deaths among women over 50.

But for younger women the benefit was found to be smaller and the associated harm greater.

Dr Wells said controversy over screening might also have diverted attention from the more pressing problem of how to prevent breast cancer.

Emotive issue

"Breast cancer is an emotive issue, and women are known to overestimate their risk of the disease," Dr Wells writes.

"The objectivity of some professionals may be clouded by the fact that they have founded their careers on supporting the use of mammography.

"Others, such as politicians, may see the potential of the issue to attract voters. As elected representatives, politicians may act in ways that reflect how they believe the public wishes them to act, irrespective of scientific evidence."

Dr Wells said the benefits of mammograms for younger women were still not proven, despite 35 years of tests.

"When trials do not give an unequivocal answer, when politicians and interest groups become involved, and when the professionals responsible for promoting the public's best interest fail to do so, objectivity is likely to suffer," she writes.

Bill Clinton: Supported screening for younger women

Pressure to lower the start age for routine breast cancer screening became intense in the US, where the Senate successfully lobbied the National Cancer Institute to recommend regular screening for women aged 40-49.

The move was publicly welcomed by President Clinton.

In the UK, all women aged 50-64 are invited for mammography every three years by the national screening programme, which was established in 1987.

A study published in the BMJ in August claimed that extending routine breast screening to women over 65 or reducing the gap between screenings could bring down the number of deaths from cancer by up to 5%.

Clinically necessary

A Department of Health spokesman said: "We don't work on political capital, we work on what is judged to be clinically necessary.

"At the moment there is no evidence that mass breast cancer screening of women aged under 50 is clinically necessary or beneficial."

Professor Gordon McVie, director of the Cancer Research Campaign, said the CRC, the Department of Health and the Imperial Cancer Research Campaign were currently undertaking a major study into the benefits of screening younger women.

"If there is any political drive to bring down the start age for routine screening, it will be balanced by logical and scientific method," he said.

"This is important research, and no decision should be taken until we get a definitive answer about the benefits or otherwise of lowering the start age."