It had hung in a townhouse for years, without anyone realising it was a precious painting by one of the most famous artists of all time.

But now, this undiscovered John Constable painting of Dedham Vale - previously dismissed as a fake - has fetched £91,000 at auction, after its true identity was finally revealed.

The 26.5cm x 43.5cm artwork of Dedham Vale, with Brantham mill and haystacks, had reportedly been dismissed by a major auction houses.

However, Sworders pictures specialist Sarah Flynn deemed the item, believed to be a preparatory oil sketch made for the Suffolk legend's famous Dedham Vale painting, worthy of further research.

She took it to Anne Lyles, the world-renowned authority on the art of John Constable - principally known for his landscapes of Dedham Vale - who declared it “an exciting new discovery”.

East Anglian Daily Times: The undiscovered John Constable painting of Dedham Vale. Picture: SWORDERSThe undiscovered John Constable painting of Dedham Vale. Picture: SWORDERS (Image: Archant)

She said simple white flecks of paint were proof of its originality.

“This is his personal shorthand for showing their position in the landscape as well, no doubt, as to show the way they catch the light depending on the time of day and position of the sun,” she said.

The picture also once passed through the hands of Leggatt brothers, art dealers who had a close link with the Constable family and handled a large quantity of sketches in oil, watercolour and pencil in the late 19th and early 20th centuries.

Sworders believes this indicates the painting may have come direct from a member of the Constable family.

“It is the dream of every auctioneer to discover a picture by one of the greats of western art.

"Dedham Vale is intimately associated with Constable’s life but the inclusion of one of his father’s mills in this picture makes it a particularly personal image, ” Ms Flynn added.

This particular sketch shows the distinctive tall tower of Dedham Church in the far right-hand distance, with a weatherboarded windmill at the village of Brantham in the middle distance.

Windmills frequently caught Constable’s attention as potential subject matter for his art.

This particular viewpoint is also believed to be the furthest east that Constable ever painted Dedham Vale.

Constable's best-known paintings include the Hay Wain from 1821 and Wivenhoe Park from 1816.