The research was first published last year, but has been re-examined following a series of protests from cancer organisations over the findings.

Now one of the world's leading medical journals, The Lancet, agrees that there is not enough evidence from large-scale trials to support breast screening.

However, cancer charities and the UK cancer screening programme disagree strongly with their verdict.

It is hoped that tumours may be spotted earlier, making treatment more likely to provide a cure.

Currently, it is reckoned that as many as 300 lives are saved a year by breast screening - and more recent estimates suggest this annual figure is climbing rapidly.

However, two Danish researchers from the Nordic Cochrane Centre in Copenhagen have re-examined the seven large-scale studies looking into the effectiveness of breast screening.

They say that the studies which support breast screening are either flawed or weak, with the only two high quality studies showing no benefit at all.

In addition, they suggest that screening may result in women receiving more aggressive treatments for cancer, increasing the number of mastectomies by approximately 20%.

They write, in The Lancet: "We hope that women, clinicians and policy-makers will consider these findings carefully when they decide whether or not to attend, or support screening programmes."

Flood of criticism

The Danish pair, Peter Gøtzsche and Ole Olsen, first voiced these criticisms last year, and provoked a flood of protest as a result.

In the light of this, they say, they have thoroughly reviewed their work - and reached the same conclusion.

"We found the results confirmed and strengthened our original conclusion," they wrote.

However, cancer organisations in the UK have repeated their attacks on the conclusions.

Stephen Duffy, an expert in breast screening from the Imperial Cancer Research Fund, said that the five studies which supported the use of mammograms should not have been excluded.

He said: "Studies in the UK and Sweden by ICRF and others have shown breast cancer screening substantially reduces women's risk of dying of breast cancer.

"Research published only in May demonstrated that women who attend regular breast screenings may reduce their risk of dying by more than 50%."

Disagreements

A spokesman for the UK Breast Screening Programme agreed: "The way Gøtzsche and Olsen classified studies was based on criteria that would not be agreed by many experts in the field.

If existing studies are too weak to support the use of breast screening, then the chances of organising large-scale replacements are slim, as these would have to involve a sizeable "control" sample who would not be screened for the purposes of comparison.

As most clinicians already feel that breast screening offers a significant benefit, it would probably be felt ethically unsound to leave so many women without it.

However, the fact that The Lancet now backs the Danish team is a significant move in supporting those who question the benefits of breast screening.

Editor Richard Horton wrote: "Women should expect doctors to secure the best evidence about the value of screening mammography.

"At present, there is no reliable evidence from large randomised trials to support screening mammography programmes."

Professor Michael Baum, from the Portland Hospital in London, says that it is now right that women should be presented with all the evidence about screening before they give their consent.

He said: "Even with the most optimistic estimates on saving lives, you would still have to screen 1,000 women for 10 years to save one life.

"If you have one significant adverse event which costs a life in this group over this period, all that benefit is cancelled out.

"The Lancet is a highly influential journal and if they are backing this review, it's highly significant."