AN expert from the Antiques Roadshow has identified an unknown landscape put up for sale by Sotheby’s for between £10,000 and £15,000 as one of the earliest works of Thomas Gainsborough, worth £750,000.

Philip Mould pieced together clues that convinced him the painting of a Suffolk view being offered at auction was created by one of Britain’s greatest 18th-century artists.

Mould, who values fine art on the BBC1 show, chose to bid for the landscape over the telephone so as not to attract suspicion.

He paid £50,000 for the painting, which Sotheby’s had described as of “the English school”. It is now valued at about £750,000.

“As soon as I saw an online image from the sales catalogue, I thought it might be Gainsborough,” said Mould. “It was the way the light was painted, and the sandy ground in the foreground, which are trademarks of Gainsborough. But we needed to do some rapid sleuthing before the sale a few weeks later.”

A vital clue for Mould, who runs an eponymously named gallery in central London, was the tiny depiction of a couple at the front of the canvas.

The expert spotted that they looked similar to a drawing Gainsborough had made of himself and his wife Margaret. Now held by the Louvre in Paris, it was created less than a year before the mystery Sotheby’s oil painting.

Working with his colleague Bendor Grosvenor, Mould set about trying to discover the painting’s provenance.

The building on the left of the picture, which shows Ipswich, a mere village in the 1740s, provided another clue.

Christchurch mansion, which is still standing, was owned by the Fonnereau family, who saw Gainsborough, a local teenager, as a promising artist and lent him money to develop his talent. “In return, Gainsborough put the Fonnereau house into his picture,” said Mould.

The BBC valuer and his team of art detectives finally turned to the painting’s records of ownership. In 1824, it was sold for £43 by Evans, a Pall Mall auction house. The vendor was George Nassau, whose father, Richard Savage Nassau, had been painted by Gainsborough in 1750 and was another family friend.

All these clues persuaded Mould to bid for the painting. The fact that its guide price was pushed up to £50,000 suggests other bidders may have also twigged that it could be a Gainsborough.

Mould, who has recently published a book called Sleuth: The Amazing Quest For Lost Art Treasures, cleaned up the picture and showed it to a group of Gainsborough experts, all of whom agreed on its authenticity.

“I simply knew it as soon as I saw it,” said Diane Perkins, director of the Gainsborough House Museum in Sudbury, Suffolk. “The clinchers were the couple, plus the family links with Fonnereau and Nassau.” Two other Gainsborough experts also confirmed it.

Now Sotheby’s could face having to compensate the painting’s vendor for drastically underestimating its worth.

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010