A CONSTABLE sold by Christie’s 18 months ago for £3,500 has been sold by their great rivals Sotheby’s in New York for a staggering £3.4million.

Lesley, Viscountess Hambleden, who married into the WH Smith family sold the painting last year after it was attributed to a follower of John Constable.

But after parts of it were cleaned, an expert from Tate Britain, Anne Lyles, confirmed it was a "lost" Constable.

It is now thought to be one of five preliminary oil sketches of Salisbury Cathedral on the Meadows considered to be of his greatest masterpieces.

The 1831 painting was bought by the Tate gallery last year for £23.1million and will going on display at Salisbury Museum at the end of next year.

The buyer who made the handsome profit is still unidentified but Ms Lyles told the BBC she confirmed it was Constable after some added brushstrokes were removed.

“It is one of the most exciting and important additions to have emerged in recent decades,” she said.

She also added that the work was “hitherto completely unknown to scholars”.

The guide price at Sotheby's in New York had been between £1.3m and £1.9m.

Christie’s, which originally sold the painting, previously confirmed that it had been attributed to a “follower” of the artist.

A spokesman said: “We are aware that Sotheby’s are offering this work as by Constable. We took the view at the time of our sale in 2013 that it was by a ‘follower of’. We understand that there is no clear consensus of expertise on the new attribution.”

The piece was sold to an anonymous buyer after a fierce bidding war, with Sotheby’s describing the result as “wonderful”.