The contraceptive pill should be made available to women over the counter after the largest study of its link with ovarian cancer showed it has prevented 100,000 deaths from the disease worldwide.

Fifty years after the pill was launched, The Lancet medical journal says it is time to remove the requirement for a doctor's prescription and offer it directly to women on demand.

Its call is prompted by "dramatic" findings from a huge study of ovarian cancer, published simultaneously in The Lancet, which combined results from 45 smaller studies in 21 countries. The findings show that taking the pill sharply reduces the risk of developing ovarian cancer and the protective effect increases with length of use.

For individual women, the reduction in risk is small – ovarian cancer is a rare disease – but because an estimated three million women take the pill in Britain and 100 million round the world, it has a large effect.

Researchers led by Professor Valerie Beral, head of the Cancer Research UK epidemiology unit at Oxford University, found that in Western countries, including the UK, 10 years of taking the pill reduced the incidence of ovarian cancer before age 75 from 12 cases per 1,000 women to eight. Deaths were cut from seven women per 1,000 to five.

The most remarkable finding, the researchers say, is that the protection lasted for more than 30 years after pill use was stopped. This is important because ovarian cancer is commoner in older women who have passed the menopause. Professor Beral said: "Worldwide, the pill has already prevented 200,000 women from developing cancer of the ovary and has prevented 100,000 deaths from the disease. More than 100 million women are now taking the pill, so the number of ovarian cancers prevented will rise over the next few decades to about 30,000 per year."

For decades after the pill was launched, it was almost constantly under a cloud of suspicion, fuelled by concerns about the social and sexual revolution that it ushered in. A stream of reports suggested the harm it caused by increasing the risk of blood clots and of certain cancers might outweigh its benefits.

Now the tide has turned. Last September, one of the longest studies of the pill, spanning 36 years and run by the Royal College of General Practitioners, showed that it reduced the risk of three cancers – ovarian, womb and bowel – in women who took it, for up to eight years. It neither increased nor reduced the risk of breast cancer but the study did show a small increase in cervical cancer. This was outweighed by the reduction in the other cancers.

That finding, published in the BMJ, has now been confirmed and deepened by the latest study in the Lancet. The co-author, Sir Richard Peto, professor of epidemiology at Oxford University, said: "Young women don't have to worry about cancer from taking the pill because the eventual reduction in ovarian cancer is bigger than any increase in other types of cancer caused by the pill."

Other research has shown that the risk of blood clots has been reduced by use of lower-dose pills and can be further reduced by excluding women with a history of blood clots, heart disease, migraine and liver disease.

The Lancet editorial says the "dramatic" findings reopen the question of whether oral contraceptives should be made more widely available in light of the latest study. "We strongly endorse more widespread over-the-counter access to a preventive agent that can not only prevent cancers but also demonstrably save the lives of tens of thousands of women," it says.

Dr Lesley Walker, Cancer Research UK's director of cancer information, said: "All women who have taken the pill or are currently taking it should be reassured by this study."