Leon Spinks flashed a broad grin, exposing the prominent gap in his front teeth, then raised a weary arm skywards as he was declared the new world heavyweight champion at the end of just his eighth professional bout.

An out-of-shape Muhammad Ali had underestimated Spinks at the Las Vegas Hilton in February 1978 and paid the price as 'Neon Leon' was given the nod by split decision in one of the most stunning upsets in the history of boxing.

A fitter and sharper Ali put down another marker on his path to greatness in the immediate rematch exactly seven months later and Spinks, who has died aged 67 in Henderson, Nevada, would never reach the same stratospheric heights again.

He was beaten up and stopped in three rounds by Larry Holmes in another world heavyweight title tilt in June 1981 and an unsuccessful pursuit of cruiserweight honours fell flat in a career which went on too long past its peak.

It was not until the mid-1990s that he hung up his gloves, finishing with a record of 26 wins, 17 defeats and three draws, but the consequences of his profession meant his fighting days did not end with his ring retirement.

Spinks, who battled arthritis and dementia, was found to have shrinkage in his brain and was largely confined to a wheelchair in his later years, while he was diagnosed with advanced stage prostate cancer in 2019.

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.

He and younger brother Michael, who went on to win world titles in the light-heavyweight and heavyweight divisions and even avenged Leon's loss to Holmes, used boxing as a means to protect themselves in their formative years.

Spinks continued to box when, as a teenager, he joined the United States 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 wounds did not deter him as he and Michael were selected to compete for the US boxing team at the 1976 Olympics in Montreal. Both came back home with gold medals, Leon at light-heavyweight and Michael at middleweight.

While his younger sibling was initially resistant to switching to the paid ranks, the older Spinks embarked on his professional journey months later with gusto, winning six times and drawing once in 1977.

It may have been that failure to beat Scott LeDoux which persuaded Ali to think Spinks was an easy world title defence. Spinks, though, would defy 10-1 odds to outpoint an ill-prepared Ali after 15 gruelling rounds.

It was close – two judges gave Spinks the decision by scores of 145–140 and 144–141 while a third plumped for Ali 143-142 – but the unexpected result meant a new undisputed heavyweight champion was crowned.

Spinks was stripped of the WBC title after favouring a return bout over a fight against mandatory challenger Ken Norton later on in 1978, and Ali would go on to reclaim the lineal heavyweight title for a record third time.

It proved to be the last victory of Ali's storied career but Spinks was adamant years later he had done enough to get the verdict, saying: "They wanted Ali (to win). It's politics. It ain't what you know but who you know."

Defeat at New Orleans' Superdome marked a prolonged decline for Spinks, who pocketed a multi-million dollar sum from his rematch against 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.

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

"I got mugged in Detroit once and I was trying to bite the guy and they came out and he stole them. It's so damn weird, people taking my teeth."

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.

While his health deteriorated, he led a more comfortable life in his advancing years with third wife Brenda in Las Vegas. He is survived by Brenda and sons Darryl and Cory.