The family had planned for restoration work to be done on the landscape setting of Iona beach by Scottish colourist SJ Peploe.
But when a board pinned to the back of the canvas was removed it revealed an entirely separate painting on the reverse side.
The previously unseen 1920s piece portrays two boys - Peploe's sons Willie and Denis - dressed in their Edinburgh Academy uniforms posing with a cricket bat. The find stunned the artist's grandson Guy Peploe, who was handed the £85,000 painting to prepare for sale at his New Town gallery. He said: "Seeing this painting of my father (Denis) and uncle (William) on the back which we didn't know was there, was incredible. It is actually the only painting I have ever seen of my father by SJ Peploe.
"My grandfather must have done this painting of my father and Uncle Willie, which is actually a lovely composition, and then perhaps thought, 'Is anyone going to be interested? Perhaps not'.
He might then turn it around so there's a new unpainted canvas and start all over again."
The two-for-one artwork gives a unique insight into the world of the acclaimed post-Impressionist, one of the four major artists who became known as the Scottish Colourists.
SJ Peploe's 'double A side' will be on show at The Scottish Gallery from tomorrow as part of its Modern Masters III exhibition and will run until July 26.