Using an oral contraceptive, often referred to as "the pill", gives long-term protection against womb cancer and the longer it is used the greater the reduction in risk, scientists said on Wednesday.

In an analysis of all available evidence, the researchers said an estimated 400,000 womb cancer cases had been prevented by use of the pill in wealthy countries the past 50 years, including some 200,000 in the last decade.

"The strong protective effect of oral contraceptives against endometrial cancer – which persists for decades after stopping the pill – means that women who use it when they are in their 20s or even younger continue to benefit into their 50s and older, when cancer becomes more common," said Valerie Beral, a professor at Britain's Oxford University who co-led the study.

"Previous research has shown that the pill also protects against ovarian cancer. People used to worry that the pill might cause cancer, but in the long term the pill reduces the risk of getting cancer."

For their study, published in The Lancet Oncology journal, Beral's team pooled data on 27,276 women with endometrial cancer in 36 studies from North America, Europe, Asia, Australia and South Africa -- using virtually all the epidemiological evidence ever collected on the effect of oral contraceptives.

They found that for every 5 years on the pill, the risk of endometrial cancer reduces by about a quarter.

In high-income countries, they found, 10 years of oral contraceptive use reduces the risk of developing endometrial cancer before age 75 from 2.3 to 1.3 cases per 100 users.

Although oestrogen levels in oral contraceptives have decreased markedly over the years, with pills in the 1960s typically containing more than double the oestrogen dose of pills in the 1980s, the study found the reduction in endometrial cancer risk was at least as great for women who used the pill during the 1980s as for those who used it in earlier decades.

The results suggest the amount of hormones in the lower-dose pills is still sufficient to reduce the incidence of endometrial cancer, Beral said.

The risk reduction was broadly in line with the number of years a woman used the pill, the study found, and was not affected significantly by other factors such as a woman's reproductive history, body fat levels, ethnicity, or alcohol and tobacco use.