To the untrained eye, it seemed like a pleasant landscape painting perfect for a spot on the wall under the staircase.Imagine one homeowner’s surprise when they discovered their painting was a genuine Constable, worth up to £1.5m and the missing piece in the development in one of his famous works.An oil sketch of Waterloo Bridge, now considered by experts to be by John Constable, has been “rediscovered” by an auctioneer Sotheby’s specialist Julian Gascoigne, who spotted it during a visit to a home in London.“That’s a very nice Constable,” he told its owners, making small talk as he got ready to leave their house.“What Constable?” replied the owner.The painting is now known to have been in a treasure trove of works owned by Camille Groult, who established the most significant collection of British art in 19th century France, and passed to his descendants.The current owner, who does not want to be named, had invited Sotheby’s to attend the house to see other paintings, unaware that their landscape was a genuine Constable.When Mr Gascoigne spotted it, he said he – with permission – tucked it carefully under his arm and hopped in a taxi to take it back to the office for careful examination. The finished painting now hangs in Tate Britain, and is famous as the work Constable hung beside JMW Turner’s seascape in the fiercely competitive 1832 Royal Academy exhibition.The sketch was known to exist and was included in a 1984 Catalogue Raisonne, but its whereabouts were never known. The story of its origins is understood to have been lost as the painting passed through the family line of Groult, who was famously private.“We can now see what Constable’s first thoughts were,” said Mr Gascoigne, of how the sketch contributed to knowledge of The Opening of Waterloo Bridge.“It’s very exciting. Working in the commercial art world, this is what you live for: the lure of finding something previously untraced.”The painting is offered for sale on December 6 at Sotheby’s, London, for an estimate of £1-1.5m. Oil sketch for The Opening of Waterloo Bridge Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. After a process which took several months, the country’s leading Constable experts and scientific examination of pigments, specialists were satisfied that it was a genuine Constable: the first preparatory sketch for his The Opening of Waterloo Bridge.