Britney Spears says in her eagerly anticipated new memoir that she had an abortion during her relationship with Justin Timberlake over two decades ago, according to an excerpt released Tuesday.

It is one of several revelations in "The Woman in Me," out 24 October, in which the 41-year-old pop star also addresses the controversial conservatorship that barred her from handling her own life and finances for 14 years.

"If it had been left up to me alone, I never would have done it," she writes of the abortion, in an excerpt published by People magazine on Tuesday.

"And yet Justin was so sure that he didn't want to be a father."

In the book, Spears describes meeting Timberlake when they were both children performing in Disney's Mickey Mouse Club.

They later began dating in their late teens, becoming one of the entertainment industry's highest-profile couples, before splitting in 2002.

According to the memoir, Timberlake "definitely wasn't happy about the pregnancy.

"He said we weren't ready to have a baby in our lives, that we were way too young," she writes.

She adds: "To this day, it's one of the most agonizing things I have ever experienced in my life."

Representatives for Timberlake, now 42 and married to actor Jessica Biel, did not immediately respond to a request for comment.

Following the split with Timberlake, Spears had two sons — Sean and Jayden — with her second husband Kevin Federline.

In 2008, after suffering a highly public breakdown, Spears was placed under a unique legal guardianship largely governed by her father, Jamie.

During a court appearance in 2021, she told a judge that the conservatorship was preventing her from removing a contraceptive IUD — despite her wanting to get pregnant.

"The conservatorship stripped me of my womanhood, made me into a child," says Spears, in another excerpt.

She also writes: "I think back now on my father and his associates having control over my body and my money for that long and it makes me feel sick."

The guardianship was scrapped in November 2021. Spears became pregnant in 2022 but later announced she had suffered a miscarriage.

In August, her third husband, Sam Asghari, said the pair were filing for divorce.