Experts at London's Sotheby's auction house revealed they have discovered an invisible ink signature made by US graffiti icon Jean-Michel Basquiat on his painting Orange Sports Figure.

The work will go under the hammer in London on Wednesday and had been expected to sell for between 3 and 4 million pounds. But it could now fetch more after the chance discovery, which showed up when it was viewed under ultraviolet light.

"Despite the scholarship that has built up around Basquiat's life and art since his tragic early death ... we are still learning new facets of how he worked," said Cheyenne Westphal, chairman of Contemporary Art Sotheby's Europe.

"Nobody else probably ever knew about this invisible inscription, and the prospect that he might have left other invisible writings on his canvases that are only visible under ultraviolet light is very exciting," he added.

Basquiat was born in Brooklyn, New York City, in 1960 and began as a graffiti artist in the late 1970s before evolving into a neo-expressionist during the next decade.

During a varied career, the artist collaborated with pop art godfather Andy Warhol, played in a band with US film actor and director Vincent Gallo, appeared in a video for New Wave band Blondie, and even briefly dated Madonna.

He died in 1988 of a heroin overdose.

Orange Sports Figure was created in 1982, a year recognised by experts as a breakthrough for the artist, and depicts a figure emblazoned with Basquiat's most iconic motif - the crown.

According to Sotheby's, the work combines an "astounding pluralistic command of art historical vernacular that synthesises graffiti, primitivism and abstract expressionism".

AFP