Boxing legend Muhammad Ali has died at the age of 74, a family spokesman has said.

The three-time world heavyweight champion, who is widely regarded as one of the greatest sportsmen of all time, died at a hospital in the American city of Phoenix after being admitted on Thursday (June 2).

He was suffering from respiratory issues with reports suggesting his breathing problems had been complicated by Parkinson’s disease, which he had suffered from since 1984 - three years after retiring from boxing.

"Muhammad Ali's funeral will take place in his hometown of Louisville, Kentucky," a statement from spokesman Bob Gunnell said.

"The Ali family would like to thank everyone for their thoughts, prayers, and support and asks for privacy at this time."

Nicknamed "The Greatest", Ali was the first boxer to capture a world heavyweight title on three separate occasions.

He won 56 of his 61 fights - 37 by knockout - before eventually retiring in 1981.

Born in January 1942 as Cassius Marcellus Clay Jr, a name shared with a 19th century slavery abolitionist, the boxer changed his name to Muhammad Ali after his conversion to Islam.

Ali shot to fame by winning the light heavyweight gold medal at the 1960 Olympic Games in Rome.

He turned professional immediately after the Games and won his first world heavyweight title less than four years later, defeating the previously unbeaten American Sonny Liston to the amazement of the boxing world in February 1964.

Liston quit on his stool after six rounds as the 22-year-old Ali triumphed to prove those who doubted him wrong.

In 1967, Ali objected to being drafted into the United States military for the war in Vietnam, a move that was widely criticised by his fellow Americans and led to a three-and-a-half-year ban from boxing.

He was subsequently stripped of his world title and boxing licence.

Ali returned to the ring following the overturning of his conviction in 1971, the year in which he suffered his first professional defeat at the hands of American Joe Frazier in the "Fight of the Century" in New York.

He went on to regain his title with an eighth-round knockout of American George Foreman in the "Rumble in the Jungle" in Kinshasa, Zaire in October 1974.

A third and final meeting with Frazier came in the Philippines in October 1975 with Ali winning the fight famously known as the "Thrilla in Manila" after Frazier’s trainer refused to allow his fighter to answer the bell for the 15th round.

Following six defences of his title, Ali lost on points against American Leon Spinks, the 1976 Olympic light heavyweight champion, in February 1978.

He avenged his defeat to regain the world title in September 1978 before suffering a TKO defeat against American Larry Holmes in the penultimate fight of his career a little more than two years later.

Ali’s career ended in December 1981 with a one-sided defeat against Jamaica’s Trevor Berbick and it wasn’t long after retiring that rumours began to circulate about the state of his health with his speech becoming slurred.

In 1984, he was diagnosed with Parkinson's syndrome, a disease that commonly results from head trauma from activities such as boxing.

Ali continued to make public appearances in spite of his illness, lighting the flame at the 1996 Olympic Games in Atlanta and carrying the Olympic flag at the London 2012 Opening Ceremony.

Asked how he would like to be remembered, Ali once said: "As a man who never sold out his people.

"But if that's too much, then just a good boxer.

"I won't even mind if you don't mention how pretty I was."