A portrait of Sir Alfred Munnings, a prominent member of the Newlyn school of artists who studied in Cornwall in the early 20th century, is thought to have been concealed by a female artist to avoid upsetting her jealous husband.
The portrait was found carefully hidden behind a painting by Dame Laura Knight.
Experts believe it was painted by Laura's husband, Harold, in 1910 while all three were part of the artists' colony at Lamorna, near Newlyn.
Sir Alfred went on to become a president of the Royal Academy and an outspoken critic of modernism.
Dame Laura is said to have adored Sir Alfred and frequently went off with him to paint on the Cornish cliffs, much to the chagrin of her husband.
Tom Rooth, Christie's specialist in British art who made the discovery, said the painting could have been hidden by Dame Laura to avoid angering her husband or as a permanent demonstration of her feelings for Sir Alfred.
"Harold Knight respected Munnings as an artist but there was no love lost between them," he said.
"It is possible Harold's dislike of Munnings led him to dispose of it and Laura rescued it without his knowledge and hid it to avoid angering him. Perhaps she wanted to ensure the portrait survived but was forced to abandon it to its fate behind another painting."
The portrait, entitled Sir Alfred James Munnings Reading, was found during a routine examination of Carnaval, a 1915 ballet scene painted by Dame Laura, during which Mr Rooth noticed the unusual appearance of two edges of stretched canvas at the back.
"I could barely believe my eyes when I removed several of the nails," said Mr Rooth. "The freshness of the colours took my breath away. It is the most important discovery in my career, to date. Why was such a beautiful and important work hidden so deliberately?"
Despite her fondness for Sir Alfred, there is no evidence Dame Laura was having an affair.
"It was not a sexual relationship as far as anyone is aware," said author Jonathan Smith, whose novel about the artists and their friends, Summer In February, is being turned into a film.
"She was in awe of him but it was really a deep devotion rather than an affair. There is something so exciting about why she went to the trouble of hiding the portrait. It is a really dazzling discovery about a massively important time for British art."
In the five years between the portrait and its concealment, Sir Alfred wed one of his fellow artists, the beautiful Florence Carter-Wood. She attempted suicide on their wedding night and killed herself two years later, causing shock and heartache within the Lamorna colony.
"Perhaps when Florence died, the portrait was covered because it evoked too many memories of a lost chapter with the group," said Mr Rooth.
"We will never know the real reason. It is a mystery, but that fact it was hidden encapsulates the intrigue, romance and complexity at the heart of the community of British Impressionist artists working in Lamorna at the beginning of the 20th century."
Sir Alfred James Munnings Reading, which passed unknowingly through several hands while hidden behind Laura Knight's Carnaval, could fetch up to £50,000 when it is sold by Christie's in December.
article copyright THE CORNISHMAN