The auction house said it expected Warhol's 1964 Shot Sage Blue Marilyn to become the most expensive 20^th century artwork when it goes under the hammer in New York in May.

The silk-screen work is part of a group of Warhol portraits of Monroe that became known as the Shot series after a visitor to his Manhattan studio, known as The Factory, apparently fired a gun at them.

In a statement, Christie's described the 100 by 100 centimeter portrait as "one of the rarest and most transcendent images in existence”.

Alex Rotter, head of 20^th and 21^st century art at Christie's, called the portrait "the most significant 20^th century painting to come to auction in a generation”.

"Andy Warhol's Marilyn is the absolute pinnacle of American Pop and the promise of the American Dream encapsulating optimism, fragility, celebrity and iconography all at once," he said in a statement.

Warhol began creating silkscreens of Monroe following the actress's death from a drug overdose aged just 36 in August 1962.

The pop artist produced five portraits of Monroe, all equal in size with different colored backgrounds, in 1964.

According to pop-art folklore, four of them gained notoriety after a female performance artist by the name of Dorothy Podber asked Warhol if she could shoot a stack of the portraits.

Warhol said yes, thinking that she meant photographing the works. Instead, she took out a gun and fired a bullet through the forehead of Monroe's image.

The story goes that the bullet pierced four of the five canvases, with Warhol barring Podber from The Factory and later repairing the paintings — the Shot series.

The Shot Sage Blue Marilyn portrait portrays her with a pink face, red lips, yellow hair and blue eye shadow set against a sage-blue backdrop.

It was based on a promotional photograph of her for the 1953 movie Niagara directed by Henry Hathaway.

Charity

At an unveiling at Christie's headquarters in Manhattan, Rotter said the portrait stood alongside Sandro Botticelli's Birth of Venus, Leonardo Da Vinci's Mona Lisa and Pablo Picasso's Les Demoiselles d'Avignon as "categorically one of the greatest paintings of all time”.

Only 14 paintings have sold for more than $100 million at auction, according to an AFP tally, although others are expected to have changed hands for as much during private sales.

The most expensive 20^th century artwork sold at auction is Picasso's Women of Algiers, which fetched $179.4 million in 2015.

The auction record for a Warhol is the $104.5 million paid for Silver Car Crash (Double Disaster) in 2013.

In 1998, Sotheby's sold the orange shot Marilyn for $17 million.

Christie's is selling the work, which will headline its spring sales week, on behalf of the Zurich-based Thomas and Doris Ammann Foundation.

All proceeds of the sale will benefit the foundation, which works to improve the lives of children around the world.

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