The American, who in just his eighth professional fight claimed a famous upset win over Muhammad Ali in February 1978, was plagued by health issues in his later years and was diagnosed with advanced stage prostate cancer in 2019.

Spinks's management company said in a statement the former US Marine died on Friday evening in Henderson, Nevada, with his wife Brenda Glur Spinks by his side, and with "only a few close friends and other family" allowed to be present due to Covid restrictions.

"Leon fought his battle with numerous illnesses resiliently, never losing his trademark smile. Showing true Spinks determination, he never threw in the towel," the statement said.

"His final fight was fought with the same skill, grace and grit that had carried him through so many lifetime challenges."

Spinks won gold at the 1976 Olympics in Montreal then rose to greater prominence a little more than 18 months later, given the nod via split decision to stun an out-of-shape Ali and claim the WBC, WBA and lineal heavyweight titles.

In one of the biggest upsets in the history of the sport, 24-year-old "Neon Leon" Spinks defied his 10-1 underdog odds with bookmakers to prevail after 15 rounds, winning 145-140 and 144-141 on two scorecards while a third went in favour of Ali 143-142.

His reign lasted exactly seven months as a sharper and fitter Ali gained revenge in an immediate rematch in September 1978, becoming the first ever three-time lineal heavyweight champion after a wide points win, the last victory of his career.

There would be no trilogy bout and though Spinks did get another opportunity for heavyweight glory in June 1981, there was no fairytale result this time as he was beaten up and stopped inside three rounds by Larry Holmes.

Spinks then dropped down to cruiserweight but was defeated by WBA champion Dwight Muhammad Qawi in 1986. He boxed on for another nine years and finished with a modest record of 26 wins, 17 defeats and three draws.

Spinks's battles in the ring contributed to his ill-health in later life. It was found he had shrinkage in the brain in 2012 before his cancer diagnosis was confirmed a few years later.

Yet fighting was all he knew from a very early age after being born into a poverty-stricken neighbourhood in St Louis, Missouri, on July 11, 1953, brought up by his mother with his father largely out of the picture.

Spinks continued to box when, as a teenager, he joined the Marine Corps, where he had several of his front teeth knocked out after being headbutted in one of his early bouts, resulting in his famous gap-toothed grin.

The loss to Ali marked a prolonged decline for Spinks, who pocketed a multi-million dollar sum from the rematch Ali but was living in a homeless shelter just a few years after retirement.

Many assumed he partied away his fortune but he insisted a lawyer's avarice was mainly to blame, while he lamented that several opportunists stole his dentally-repaired teeth over the years.

"They love my smile so much they steal my teeth. Sometimes I leave them in the hotel," he told the Observer. One time they were stolen by a maid or somebody who got a key to my room. People do some crazy stuff.

Twice divorced, Spinks had three sons. His eldest, Leon Jr, was shot to death in his car aged 19 in 1990, while his youngest, Cory, is a former undisputed welterweight champion who also won a world title at light-middleweight.

Spinks's younger brother Michael reigned as undisputed light-heavyweight champion for a period in the 1980s, before moving up to heavyweight, where he won the IBF title and avenged Leon's loss to Holmes.

Australian Associated Press