Video report by ITV News correspondent Paul Davies.
They were opponents in the ring and close friends outside it.
But the British boxer Joe Bugner recalls that his first meeting with the already great Muhammad Ali did not initially bode well for their relationship.
"So you're the white boy that wants to make a name are you?", he recalls Ali telling him when they met in 1969. "Well, let me tell you something, white boy: damn, you are ugly! Your mother must have cried when she had you."
"I said 'That's it!'," says Bugner of that first conversation. "I said, 'Let me tell you something...You haven't met my sister.'"
"We just clicked," he told ITV News as he looked back on his long friendship with the superstar fighter and activist.
Indeed, Ali was quick to champion Bugner as he rose from an upstart from Cambridgeshire to a heavyweight professional contender in the ring.
They fought each other twice in professional bouts, with Bugner lasting more than ten rounds each time - though Ali won both their fights.
Meanwhile, the men had cemented a solid friendship outside of their professional rivalry, ultimately becoming neighbours in Beverly Hills.
Bugner, who now lives in Australia, remembers Ali as equally the "most loving man" and an paralleled fighter who is never likely to be matched.
"To me Muhammed Ali was the greatest boxer, heavyweight fighter on the planet," he said. "Then and now and forever."