Women taking the most common form of hormone replacement therapy in Britain are twice as likely to develop breast cancer, the world's largest study on the treatment's risks has concluded. An extra 2,000 British women a year are developing the disease because of HRT, researchers warned.

The risks associated with the combined oestrogen-progestogen HRT are far higher than previously thought and begin much earlier than doctors had assumed.

More than 750,000 women in Britain are currently taking combined HRT tablets, patches or implants to treat symptoms of the menopause. They can also help to prevent the bone disease osteoporosis.

Breast cancer is the biggest killer of women in the country, with 1,000 deaths a month from the disease.

Last night, the Government's Committee on Safety of Medicines (CSM) sent e-mails and faxes to every doctor and health professional in the NHS, informing them of the new findings, published today in The Lancet.

The CSM advice stops short of telling doctors to take women off combined HRT, but it says they should be told about the increased risk and checked at least once a year for any problems.

Dr Mary Armitage, who chairs the CSM's steering group on HRT, said: "The breast cancer risk does come through much earlier than we thought, but for short-term use for the relief of menopause symptoms, for most women the benefits will outweigh the risks. For long-term use, women need to understand the increased risks and perhaps talk to their doctors about other strategies."

While officials stressed that this was "not a medical emergency", GPs are bracing themselves for an influx of worried women to their surgeries this week.

Manufacturers of HRT products - now a multimillion-pound market - are expected to see share prices slump.

The research in The Lancet involved more than a million women in Britain, making it the world's largest study into HRT and breast cancer. A team led by Professor Valerie Beral, director of the Cancer Research UK epidemiology unit, spent 10 years tracking 1,084,110 women aged between 50 and 64 - equivalent to a quarter of all British women in that age group.

Smaller studies have suggested that women on combined HRT are 26 per cent more likely to develop breast cancer. But the Million Women study, as it is called, concluded that combined HRT users are 100 per cent more likely to get the disease.

It is already known that oestrogen-only HRT preparations, less popular than combined products, carry a 30 per cent increased risk of breast cancer. The risk for both types begins within a year of starting an HRT course, and increases in proportion to the duration for which women are taking the treatment.

A typical woman spends about five years on HRT, although some take it for up to a decade. There is no known difference in risk between pills, patches and implants.

The "Million Women" findings have been eagerly awaited by the scientific community, but will plunge the 1.5 million women who take HRT in the UK into a medical quagmire of statistics, odds ratios and risk analysis.

Women taking oestrogen-only HRT may run a lower risk of developing breast cancer, but it a higher risk of endometrial cancer - in the lining of the womb. For many menopausal women, coming off HRT altogether means enduring years of debilitating symptoms such as hot flushes, night sweats, heavy and irregular periods, headaches and muscle pain.

Professor Beral said: "The risks are substantially greater than previously thought, but the evidence is overwhelming. Our main conclusion is that it is really up to individual women and their doctors to try to figure out what their risks and choices are.

"There is no simple answer, because the benefits of relief from menopausal symptoms may outweigh the risks for many women."

For every 1,000 women who take HRT for 10 years from the age of 50, there will be an extra 19 cases of cancer among users of combined products and an extra five among oestrogen-only patients. Experts stressed that the increased risk stopped when women ceased taking HRT, and after five years the risk was the same as for someone who had never been on the treatment.