Previously only known from a 1910 print and unrecorded for half a century, this 20in x 2ft 6in (50 x 75cm) oil by John William Godward (1861-1922) will be offered by Lawrences in Crewkerne on October 12.
Painted in 1909, Summer Idleness: Day Dreams had passed through Christie's in London in 1937 (when it made just under £70), but it is being sold by the daughter of a lady who purchased it from Harrods in 1957 for £100.
"It's a wonderful find," says Lawrences' specialist Richard Kay, who expects it to bring £150,000-200,000. "The owners knew that they owned a fine picture but had no idea that its whereabouts had been unknown for well over half a century."
Dr Vern Swanson, the American authority on Godward, whose monograph of the artist's work The Eclipse of Classicism lists dozens of Godward's pictures, has recently assessed the picture and declares it to be a revelation, saying that it is "one of the most sensitively coloured paintings in Godward's oeuvre". He will now include it with a bigger illustration in an updated edition of his book and is delighted to learn of its existence.