The diminishing risk disappears after 10 years and thereafter former users are at no increased risk of breast cancer compared with women who have never taken the Pill, according to the data to be published later this week.
An unexpected finding is that breast cancers in women who have taken the Pill are less likely to have spread beyond the breast - and are potentially more curable - than in women who have never taken it. Scientists cannot explain this.
In addition, the absence of a long-term risk (after 10 years) for former users is true regardless of how old women were when they began taking the Pill; how long they took it for; and what type of Pill they took.
The good and bad news elements of the findings may do little to help women reach a conclusion in the long-running debate on the safety of the Pill, but scientists hailed the four-year study - the largest research to date of the links between oral contraceptives and breast cancer - as ground-breaking.
Dr Valerie Beral, head of the Imperial Cancer Research Fund's Epidemiology Unit which reviewed 54 international studies involving 53,000 women with the disease, said that the new findings - to be published in The Lancet - highlighted the age at which women stopped taking the Pill as a "crucial" factor.
Because the incidence of breast cancer is lower in young women, the estimated increase in the number of breast cancers diagnosed up to 10 years after stopping is smaller the younger women are when they last used the Pill.
The data shows that in 10,000 women who used the Pill from the age of 40 to 44, about 260 breast cancers would be diagnosed in the period from starting use to up to 10 years after stopping, compared to 230 cancers in non-users.
In 10,000 women who used the Pill from age 25 to 29, the corresponding figures are 49 breast cancers compared to 44 cancers in non-users. In 10,000 women who used the Pill from ages 20 to 24, the figures are 17.5 and 16, and for women who used it from ages 16 to 19, they are 4.5 and 4.
Dr Beral said: "We now know that more than 10 years after stopping the Pill, women are not at an increased risk."
Sue Wood, head of the Post-Licensing Division of the Medicines Control Agency, said yesterday that the risks and benefits of the Pill would be reviewed in light of the new findings, but it is unlikely that existing advice on prescribing it would change substantially.
Family planning and birth control support groups have urged women not to panic over the new findings or to stop taking the Pill.
Previous "scares" - linking it to breast cancer or blood clots - have resulted in numerous unplanned pregnancies and abortions.
Anne Weyman, chief executive of the Family Planning Association, said: "The study shows a very small additional risk for younger women which increases with age. These risks have to be balanced with the Pill's protection against other cancers such as ovarian and endometrial [womb] cancer."
The Birth Control Trust said that the cost of the last Pill panic in October 1995 over an increased risk of clots with modern Pills, was "devastating."
For the majority of women under 35 the advantages of the Pill are far greater than the disadvantages of serious, but rare complications, a spokeswoman said.
t Pill users seeking information could call the FPA's contraception education service helpline on 0171 837 4044.Reuse content