The researchers said statins could affect hormone regulation in the body, especially as the study found women on the drugs were significantly more likely to suffer cancers driven by the hormone oestrogen
Women who take statins for more than a decade face double the risk of contracting the most common type of breast cancer.

Alarming findings raise new concerns over the long-term safety of a widely prescribed medicine in the UK.

Previous studies have suggested the cholesterol-lowering drugs, used by an estimated eight million men and women, can reduce the risk of certain cancers - including the breast form of the disease.

However, most research looked at patients who had only been on them for five years or less.

The latest findings identified invasive ductal carcinoma (IDC) which starts in the ducts of the breast before spreading inwards. It accounts for around seven out of ten breast cancer cases.

The experts at the Fred Hutchinson Cancer Research Centre in Seattle, US, also found the chances of getting invasive lobular carcinoma, which accounts for ten to 15 per cent of breast cancers, went up almost 2.5 times in some women on statins long-term.

Around 48,000 women in Britain are diagnosed with breast cancer each year, equal to around 130 a day. A woman has a one in nine chance of developing the disease at some point in her life.

The reasons why the anti-cholesterol pills might stimulate cancer growth are unclear.

The researchers said one explanation may be that statins affect hormone regulation in the body, especially as the study found women on the drugs were significantly more likely to suffer cancers driven by the hormone oestrogen.

They said it's possible that while short-term use does appear to have a protective effect against breast cancer, in the long-run statins may damage certain chemical pathways that lead to growth of tumours.

The report found: 'As more women are taking them and for longer durations it is possible we will observe effects that prior studies could not detect.'

Last night, leading UK cancer bodies called for urgent research to clarify the risks to women.

But they urged patients on statins not to stop taking them without consulting their GP.

Sally Greenbrook, from the charity Breakthrough Breast Cancer, said: 'Any study suggesting a potential link between statins and breast cancer risk should not be taken lightly. But these drugs are extremely effective at reducing the risk of cardiovascular disease.'

Jessica Harris, of Cancer Research UK, said: 'There's been a huge amount of research into the link between statins and cancer.

'But so far there's no conclusive answer, with some studies showing a reduced risk, some no link, and others showing a raised risk.'

Statins have also emerged as a major weapon against heart disease in the last 20 years.

The latest research, published in the journal Cancer Epidemiology Biomarkers and Prevention, examined how long-term statin use affected breast cancer risk in women aged between 55 and 74.

The researchers studied just under 2,000 women diagnosed with either IDC or ILC between 2000 and 2008 and a separate group of 902 women of a similar age profile but who were free of cancer.

Around 370 men a year in the UK are diagnosed with breast cancer - but the latest research did not investigate the cancer risk of men taking statins.