A new book on the life of Muhammad Ali claims the iconic boxer had a ravenous sexual appetite - and would have sexual contact with girls as young as 12.

US author Jonathan Eig devotes an entire chapter in Ali - A Life to the former heavyweight champion and activist's endless affairs and one-night stands, calling 'The Greatest' a sexual predator who would bed anyone from Hollywood stars to chambermaids.

According to Eig Ali's "appetite for affection" would prove insatiable, "opening him to relations with countless girls and women, including four wives".

"Black women, white women, young women, old women, Hollywood actresses, chambermaids: Ali didn't discriminate," Eig writes.

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"Everyone close to the fighter knew his proclivities."

Belinda Boyd, Ali's second wife, claims the boxer "loved sex and was a sex addict".

Boyd, who married Ali at 17 and changed her name to Khalilah after converting to Islam, split from the boxer nine years later.

"Ali had a dark side. An evil side," Boyd told Eig, revealing how he would manipulate her to such an extent that she not only booked hotel rooms for his mistresses but agreed not to disturb him when he was with other women.

The book claims Ali physically abused two of his four wives and that he raped girls as young as 12.

There's also an extensive recounting of the children he allegedly fathered out of wedlock.

The controversial and outspoken Ali, who visited New Zealand in 1979, died in June last year aged 74 after suffering from Parkinson's disease since 1984.

He won 56 of his 61 professional fights and is considered one of the most significant and celebrated sports figures of the 20th century.