Francis Bacon's Study for a Pope I is valued at £25m-35m ($39.3m-55m) ahead of a contemporary art evening sale at Sotheby's London on July 1.
The work is one of a famous series of paintings Bacon produced in homage to Diego Velazquez's Portrait of Pope Innocent X (circa 1650).
Bacon said of his fascination with the work: "I think it is one of the greatest portraits that have been made, and I became obsessed by it.
"I buy book after book with this illustration in it of the Velazquez Pope, because it just haunts me, and it opens up all sorts of feelings and areas of - I was going to say - imagination, even, in me…I think it's the magnificent colour of it."
Bacon is among the world's most valuable artists. His Three Studies for a Portrait of Lucian Freud held the auction record for a piece of art, after selling for $142.4m in 2013, until it was eclipsed by Picasso's Women of Algiers earlier this year.
Gerhard Richter's A B, Brick Tower is expected to make £12m-16m ($18.8m-25.1m).
The work dates to 1987 and is inspired by a visit to London. It featured in Richter's first exhibition there in 1988.
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