The contraceptive pill protects women against some cancers for more than 35 years after they stop taking it, the longest study ever carried out into the health risks has found.

In recent years there have been fears that the combined pill raises cancer risk, but new research by the University of Aberdeen found that, for ovarian, endometrial and bowel cancer it actually has a strong preventative effect.

Although there was a slight increase in risk for breast and cervical cancer, the study showed it was only a temporary rise and the danger vanished a few years after stopping contraception.

The Oral Contraception Study was established by the Royal College of General Practitioners in 1968, seven years after the pill was first introduced into Britain on the NHS. It has followed 46,000 women ever since to monitor the long-term impact.

It found that taking the pill for any length of time lowered the cases of bowel cancer by 19 per cent, endometrial cancer by 34 per cent and ovarian cancer by 33 per cent.

It means that for every three women who would have developed ovarian or endometrial cancer, one has been protected by taking the pill. For bowel cancer around one fifth of cases were prevented through oral contraception. Around 35,000 women are diagnosed with the three conditions each year.

"These results from the longest-running study in the world into oral contraceptive use are reassuring," said lead author Dr Lisa Iversen.

"They provide strong evidence that most women do not expose themselves to long-term cancer harm if they choose to use oral contraception; indeed, many are likely to be protected.

"Because the study has been going for such a long time we are able to look at the very long term effects, if there are any, associated with the pill.

"Specifically, pill users don't have an overall increased risk of cancer over their lifetime and that the protective effects of some specific cancers last for at least 30 years."

The combined contraceptive pill works by tricking the body into thinking it is pregnant by delivering a surge of two hormones - oestrogen and progesterone - which cause changes in the reproductive system to prevent conception.

But oestrogen is known to feed some tumours so there were fears that taking the pill could raise the risk of cancer in the long term. However the new research found the although the risk of breast cancer rose by four per cent for women taking the pill, it disappeared five years after stopping contraception.

Professor Helen Stokes-Lampard, Chair of the Royal College of GPs, said: "Millions of women who use the combined oral contraceptive pill should be reassured by this comprehensive research that they are not at increased risk of cancer as a result - and that taking the pill might actually decrease their risk of certain cancers.

"This is not to advocate that women should be given the pill as a preventative measure against cancer as we know that a minority of women do have adverse health effects as a result of taking the pill.

"Ultimately decisions to prescribe the pill need to be made on a patient by patient basis, but this research will be useful to inform the conversations we have with our patients when discussing various contraceptive options that are available."

Around three million women are thought to take the combined contraceptive pill and charities said that it provided more evidence that there were few long-term health effects.

Emma Shields, Cancer Research UK's health information officer, said: "This long-running study adds to the evidence that the pill impacts a woman's risk of cancer. Previous large studies have shown that the pill reduces the risk of ovarian and womb cancer but increases the risk of cervical and breast cancer.

"But, we also know that once a woman stops using the pill these increased risks start to fall back down while the reduced risk of ovarian and womb cancer continues.

"If you are considering starting or stopping the pill, you should talk to your doctor."

The study was published in the American Journal of Obstetrics and Gynaecology.