More than two dozen sketches by 18th century artist Thomas Gainsborough have been discovered at Windsor Castle.
Experts had attributed the works, part of the Royal Collection, to another artist, Sir Edwin Landseer, for more than a century, the BBC reports. They had been kept in a bound volume in the Print Room at the castle since 1874.
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It wasn't until later re-examination that she realised one of them was a study for a 1748 Gainsborough painting called Cornard Wood.
Further study revealed they were all his work.
"It's thrilling," she told the BBC. "It's the very best collection of Gainsborough's early drawings in existence."
Rosie Razzall, curator of prints and drawings at the Royal Collection, agreed, saying: "We're very much convinced that these are an important group of early drawings by Thomas Gainsborough.
"It's an extremely significant discovery. It means we are able to re-appraise the early work of Gainsborough."
Both women noticed that the study for Cornard Wood bares striking similarities to another Gainsborough work in the British Museum, with noticeable fold lines and oil stains that also offer clues about how the artist worked.
Lines on the drawing also reveal his "squaring up" technique for transferring the study composition to the main painting, which was "a perfect match" when superimposed onto the final version.
Art historian James Hamilton told The Guardian that relatively few of Gainsborough's early landscape drawings have survived. The new finds revealed the artist "at his most inquisitive and energetic", he added.