A new contraceptive pill that can cut the risk of breast cancer could be on the market in the next five years, scientists said today.

The pill works by stopping periods and could signal an end to the symptoms of pre-menstrual tension. It could also help women suffering from conditions like endometriosis and cut the risk of thrombosis in those who are older, overweight and who smoke.

The current combined pill, which is used by more than two million women in the UK, carries an increased risk of breast cancer. It is made up of the hormones oestrogen and progesterone, with the progesterone believed to lead to increased cancer risk.

The new pill works by blocking the effects of progesterone and is based on compounds used in the abortion pill, RU486. The new pill works with much lower doses than in RU486 and blocks progesterone before a pregnancy even occurs.

David Baird, professor of reproductive endocrinology at the University of Edinburgh, who is leading the research, said that current animal tests showed that the pill inhibits the introduction of breast cancer.

"On theoretical grounds, there is no reason to suspect that this should increase the risk of breast cancer, in fact, it might even decrease it," he said.

"Women who don’t take any pills have quite a high risk of developing breast cancer. A lot of factors contribute to that - one is that they have repeated monthly cycles. If you reduce the cyclical exposure of the ovary to the ovarian hormones oestrogen and progesterone, you should reduce the risk of breast cancer.

"The same is not true for the combined pill, which in theoretical and epidemiology studies increases the risk of breast cancer."

Professor Baird said that larger-scale studies were needed to examine the full benefits of the new pill as well as any possible side effects. But he said the pill had been tried on two groups of around 90 women with positive results and very few side effects.

The pill is one of a group of progesterone receptor modulators that block the effects of progesterone - a hormone that prepares the body for pregnancy. The compounds used were first created in 1980 and then marketed as an abortion pill in 1988, he continued. Studies carried out in Sweden, Chile, New York and Edinburgh had shown the ability to use the compounds as contraceptives.

Early findings in patients with advanced breast cancer had also suggested that the compounds used "may be helpful", Professor Baird said.

Professor Baird said there had been difficulty in funding the projects because of attitudes surrounding the use of the compounds as an abortion pill. US President George Bush’s administration had also hampered studies into such drugs, he added.

"If you could develop a pill which significantly reduces the risk of breast cancer, it would be a bestseller," he continued.

The findings were unveiled at a press conference to mark the 50th anniversary of the pill. According to the Family Planning Association (FPA), about 3.4 million women in the UK - 25 per cent of all women of child-bearing age - take either the mini or combined pill, with two million of those taking the combined pill.

Anna Glasier, director of sexual and reproductive health research at the London School of Hygiene and Tropical Medicine, said that the idea that it was unnatural to stop periods was "completely wrong". She added that "more and more women are choosing methods that stop their periods".

Jeremy Hughes, chief executive of the charity Breakthrough Breast Cancer, said it was "too early" to say what effect the new pill will have on breast cancer risk. He added: "Large, long-term studies are needed in order to determine the effects, if any.

"Women who currently take the contraceptive pill have a slightly increased risk of breast cancer. However breast cancer in women under 40 is rare, whether they use the pill or not.

"It’s important to keep in mind that the pill offers benefits, like preventing unplanned pregnancies and reducing the risk of ovarian and endometrial cancer. Women should consider the risks and benefits when deciding to use the pill and speak to their GP or family planning clinic for advice."

• The headteacher of a Catholic school today described how she became the victim of a national hate campaign after teaching pupils about contraception.

Diana Vernon, of Woldingham School in Surrey, a foundation school for the order of nuns known as the Society of the Sacred Heart, where boarders pay £21,000 a year, said she had been bombarded with hostile emails and poison letters after giving lessons on birth control to 14 and 15-year-old pupils. She made a formal complaint to police after one of the emails accused her of "child abuse".

Mrs Vernon said: "The girls are given a very clear sense of what the Catholic Church requires, but we also make sure that theys are aware of the options. They can leave here and make an informed moral choice for themselves," she said.

The emails were received following a mention of the school and its email address in the e-newsletter of the anti-abortion group UK LifeLeague. The group claims to have 17,000 subscribers in Britain and the US.

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