Susan Foister, a senior curator at the National Gallery in London, used the latest X-ray technology to "strip down" the picture by the 18th-century British artist. The X-ray revealed a completely different painting underneath.
Ms Foister is organising the exhibition "Young Gainsborough" which opens at the National Gallery tomorrow .
She said yesterday that she had become worried about one picture lent for the exhibition, Couple in a Landscape, which she had viewed often at the Dulwich Picture Gallery in south-east London.
"I was suspicious of the Dulwich picture," she said yesterday. "The skirt of the woman didn't look like Gainsborough's dresses. Normally in his paintings you see the folds beautifully portrayed and the light falling on the satiny-looking skirts. In this one the folds aren't neat. There's a lack of logicality in the way the highlights are shown.
"I thought it would be interesting to check it out, so I had the painting brought in early and X-rayed it. The X-ray that came out was quite extraordinary. For a start, Gainsborough had worked on it the other way up, and when we turned it round, we saw there was a painting of the head and shoulders of a woman in a hat and dress. "We could see that the lips of this woman were partly shading the dress in the later picture."
Ms Foister says that the earlier picture is "without doubt a Gainsborough", though she does not know who the subject is. She says that it is possible that Gainsborough was commissioned to do the earlier painting and the commission was then cancelled. In the interests of economy, Gainsborough might have used the same canvas for the later work.
"It's an impressive portrait of a strong-jawed woman in a beautiful dress," says Ms Foister. "And it's another painting by Gainsborough from a very interesting period in his career, the early period. It's something art historians will be rather excited about."