The new policy from the American Medical Association (AMA) comes as the UK's screening programme assesses the value of screening from 40 in a major study.

At the moment, women in the UK get mammograms to check for the possibility of cancer every three years between the ages of 50 and 64.

In the US, a programme of annual screenings starts at 50.

But recent studies published in The Lancet medical journal showed that screening from 40 can reduce deaths from the disease, and researchers said the starting age should be lowered accordingly.

Research also shows that the types of breast cancers younger women develop tend to be more aggressive than those affecting older women.

Cancer incidence in under-50s

The AMA's House of Delegates approved the policy change as it concluded its annual meeting in Chicago.

The association joins the American Cancer Society and the American College of Radiology in adopting the policy.

Referring to the recent studies, it recommended screening every one to two years depending on whether or not the woman had a family history of breast cancer.

It also referred to figures from the cancer society saying that women in their 40s accounted for 18% of all new breast cancer cases in the US.

And it encouraged the medical profession as a whole to develop a unified policy on the matter.

'Cut-off point is too early'

Delyth Morgan, chief executive of the Breakthrough Breast Cancer charity, said the upper age limit was too low.

"Things like this are opening up the debate in this countries, but there are other priorities as well - we would like to see women between 65 and 69 screened."

And she said women should be sceened every two years instead of every three.

Julietta Patnick, national co-ordinator of the NHS Breast Screening Programme, pointed to the success of current screening and said she looked forward to seeing the AMA's reasons for adopting the policy.

"We are very interested in the recommendation made by the AMA and are keen to see the evidence on which this decision has been based," she said.

"Screening women under the age of 50 in the UK is an issue that the NHS Breast Screening Programme is taking a great interest in.

"We are currently undertaking an Age Trial study that involves inviting 65,000 women in their 40s."

She said interim results of the trial may be available by 2003.

Controversial screening programme

In 1997 the US National Institutes of Health considered introducing the lower age, but concluded there was not enough evidence to support such a move.

The decision to have a screening programme at all has proved controversial in the UK, with some doctors saying the money would save more lives if invested into research into chemotherapy drugs and radiotherapy techniques.

In the UK, around 15,000 women a year die from breast cancer, although the screening programme estimates that 1,250 a year are saved through its efforts.