Hormone replacement therapy (HRT) increases the risk of breast cancer by a third, the most comprehensive study ever undertaken has shown.

New research by Oxford University scientists found the long-term chances of developing the disease for women on the most common form of treatment is double what was previously feared.

The experts last night called on health leaders and GPs to emphasise the risks to patients, following an increasing tendency to downplay the link in recent years.

Approximately one million women in the UK take the drugs to ease symptoms of the menopause such as hot flushes and night sweats.

There was a huge increase in uptake during the 1990s, but this roughly halved in the early 2000s after the first evidence suggesting a link to cancer emerged.

Numbers have crept up again, however, amid concerns that many family doctors no longer properly set out the cancer risk before prescribing the drugs.

Published in The Lancet, the study reviewed all the worldwide evidence on HRT and cancer and included data from more than 108,000 users who had developed the disease.

It found that while the general risk of breast cancer between the ages of 50 and 69 for women who have not taken HRT is 6.3 per cent, those on oestrogen plus daily progestagen - the most common form used by more than half of patients - for five years have an 8.3 per cent risk.