Francesca Young, modern and contemporary art specialist at Tenants Auctioneers, said it was “really exciting to have it here for sale”.
“We didn’t know it was coming in,” she said.
“Riley bought it from the artist’s end of year show and the family have had it ever since, so it’s very nice to have something like that that’s never been seen before.”
Bradford-born Hockney is considered one of the key figures in the 1960s pop art movement.
He is famous for his vibrant depictions of Los Angeles swimming pools, portraits, and large-scale Yorkshire landscapes.
His most iconic works include A Bigger Splash (1967), Portrait of an Artist (1972), Mr and Mrs Clark and Percy (1971), and A Bigger Grand Canyon (1998).