A painting by John Constable is expected to sell for up to £500,000 - a year after it was first valued by another auction house at £500.
East Sussex-based auctioneer Gorringes said originally it believed the painting, Storm Clouds over Hampstead, was done by a follower of Constable.
However, Sotheby's experts have now said it was one of Constable's.
Gorringes have refused to comment on the matter. Sotheby's said Constable copies were often hard to identify.
The picture, which sold at Gorringes, based in Lewes, for £24,000, is being auctioned again by Sotheby's on 8 July.
Emmeline Hallmark, head of British Paintings at Sotheby's in London, said the 16in by 27in (40.6cm by 69.2cm) picture showed a "beautiful" stormy sky.
"On the one hand there are brilliant blue skies and on the other hand you have deep, dark, atmospheric thunderclouds," she said.
"We have also got white fluffy clouds in the middle of the composition."
The Gorringes catalogue for its auction on 10 June 2008 calls it "Passing Form - Clouds" and estimated the oil painting's price at between £500 and £800.
It was described as "rather dirty, painted on to some sort of thick mill-type board ... some general scuffing and scratching in some parts into the paint".
Ms Hallmark said: "Constable was widely faked and copied.
"We get a number of inquiries each week about pictures which people think are by John Constable and are not.
"The process to identify a work as by the artist himself is a very long one and involves a number of experts and in-depth analysis of the technique and the medium."
She said that the person who bought it from Gorringes had a "gut instinct" about its authenticity.
"The owner loves the picture and that is the most important thing," she said.
"They have decided to sell it but only at a certain price below which they are happy to keep it and have it back at home."
The estimated sale price is between £300,000 and £500,000 but it could sell for more.
"It is hard to predict how many other people will believe it is worth rather more than that so it is an exciting prospect," said Ms Hallmark.