Gainsborough’s master, Francis Hayman, must be turning in his grave. Almost 300 years after Hayman took revenge on his estranged wife by drastically ripping her image from his portrait of them in happier days, art experts plan to reunite the two.

Hayman’s other half was spotted in an auction house in the United States by Philip Mould, an art dealer who has a record of discovering lost masterpieces.

No one knew Hayman’s wife was even missing from the painting, which hangs at the Royal Albert Memorial Museum and Art Gallery in Exeter, Devon.

Mr Mould bought the picture, described in the auction as Circle of Hogarth, because he was intrigued by its quality. But when he removed a heavy layer of overpaint he found that a brown lump at the bottom left-hand side of picture was in fact the knee of a man wearing brown trousers.

He detected the hand of Hayman in the portrait, and after a visit to the Royal Albert Memorial Museum, realised the extent of his find.

Hayman was one of the leading painters in 18th-century Britain and his pupils included Thomas Gainsborough. This year, the Tate paid £1 million for his painting Portrait of Samuel Richardson.

He will be reunited with his wife for the first time in three centuries tomorrow at Mr Mould’s new gallery in Central London.

Together, the portraits make sense. Hayman’s half shows him holding his palette. Behind his wife, stands a painting on an easel. However, the two halves are no longer exact. The artist scaled back his half of the portrait, to create a more compact image, losing a couple of inches from both the top and bottom.

The Exeter museum hopes that they will remain together permanently on its walls and has applied for grants to buy the newly discovered section for an undisclosed sum.

Nothing is known about Hayman’s wife, not even her Christian name, and quite why he fell out with her is equally mysterious. It seems that he did soon after the portrait was painted in 1735. “All we know is that she existed. It’s almost as if there was a conspiracy to expunge her from the records,” Mr Mould said.

But Hayman does show tenderness for his wife in the painting. Mr Mould said: “There is certainly a feeling of affection and delight in the way her facial features and hands are treated. There’s an informality that you don’t find in his orthodox portraits.”

Hayman was somewhat unlucky in love. It seems that he did not like his second wife either, never bothering to paint her, as far as historians know.

Mr Mould paid tribute to his researcher, Bendor Grosvenor, for spotting the advertisement for the auction.

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