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.