A Francis Bacon triptych once owned by the celebrated author Roald Dahl is expected to fetch up to 70 million US dollars (£56m) at auction.
Three Studies For A Portrait Of George Dyer (1963) marked the first time that Bacon painted his lover and muse.
Dyer would go on to appear in at least 40 of Bacon's paintings, following his first appearance in the "masterful triptych".
James And The Giant Peach and Charlie And The Chocolate Factory author Dahl bought the portrait and three other Bacon works between 1964 and 1967.
The writer had been a great admirer of Bacon for many years but was only able to start collecting his work after his own career took off.
The work, which is being sold from a French collection, is expected to fetch between 50 million and 70 million US dollars (£40m to £56m) at Christie's Post War and Contemporary Art sale in New York on May 17.
It will mark the first time the portrait has come to auction.
Christie's post-war and contemporary art deputy chairman Loic Gouzer said the triptych was completed within the first three months of Bacon meeting Dyer, who committed suicide in 1971.
"This powerful portrait exemplifies the dynamism and complex psychology that the artist is most revered for.
"George Dyer is to Bacon what Dora Maar was to Picasso. He is arguably the most important model of the second half of the 20th century because Dyer's persona as well as physical traits acted as a catalyst for Bacon's pictorial breakthroughs.
"The Francis Bacon that we know today would not exist without the transformative encounter that he had with George Dyer," Mr Gouzer said.
Another Bacon painting, featuring his friend and fellow artist Lucian Freud, became the most expensive artwork ever sold at auction when it fetched 142 million dollars (then worth £89 million) in New York in 2013, although Picasso's masterpiece Les Femmes d'Alger later smashed the record.
Dublin-born figurative painter Bacon died in 1992.
:: Three Studies For A Portrait Of George Dyer (1963) will be on view at Christie's London until March 8.