The multi-coloured portrait of "The Greatest" will be offered during Christie's Post-War and Contemporary Art Evening Auction on February 20

Muhammad Ali belongs to a set of ten silkscreen portraits of sports giants of the 1970s, including tennis player Chris Evert, golfer Jack Nicklaus and soccer player Pelé.

This series, completed by Warhol between 1977 and 1979, came to life at the bequest of art collector and sports enthusiast Richard Weisman. At the time, the Pop artist was unfamiliar with the athletes that Weisman selected for the series, with the art collector even saying that "Andy didn't really know the difference between a football and a golf ball."

Warhol photographed every subject of the series with a Polaroid Big Shot camera, before translating the photographs to acrylic and silkscreen ink on canvas. The Pop artist met the legendary boxer, who had then defended his title an extraordinary nine times, at his training camp in Pennsylvania in August 1977.

As Christie's pointed out, the encounter between the two men was quite eventful, with Ali asking Warhol, "white people gonna pay $25,000 for my picture?!"

As did several canvases of the Athletes series, Muhammad Ali appeared in magazines and billboards across America to promote sportswear, cars and even breakfast cereals, prompting Warhol to declare that "the sports stars of today are the movie stars of yesterday."

Now hitting the auction block at Christie's London, the colourful portrait of The Greatest is expected to sell for between £3 million and £5 million (around $3.9 million and $6.5 million).

Muhammad Ali will directly be offered from the private art collection of Weisman, which also includes portraits from the Athletes series of Kareem Abdul-Jabbar (estimated between £220,000 and £280,000), Rod Gilbert (estimated between £200,000 and £300,000) and Pelé (estimated between £300,000 and £500,000).

Last November, another set of Warhol's Athletes series was offered as individual lots at Christie's in New York, amassing a total of $15,014,000.

Ahead of Christie's Post-War and Contemporary Art evening sale, highlights of the auction will go on public view from February 8 at Christie's King Street galleries in London.

See also: US$600,000 Worth Of Salvador Dali Sculptures Stolen In Sweden