A painting could fetch millions of pounds at auction today after "100 years of dirt and old varnish" were removed - revealing it to be a 17th-century masterpiece.
The Rubens painting, which had spent 139 years in a family collection, is valued at £2.5m to £3.5m and will go under the hammer at Sotheby's in London tonight.
Portrait Of A Lady was exhibited at the Royal Academy in London in 1902 as a work by the Dutch painter, but its origins had been "forgotten about", Andrew Fletcher, Sotheby's head of department for Old Master paintings, said.
Image: The painting will be offered with an estimate of £2.5m on 29 July
It was sold for £78,000 in 2017 but was catalogued at the time as being from Rubens' workshop, meaning it was thought to have been painted by one of his assistants around 1625, possibly overseen by the Old Master himself.
The anonymous buyer - who is now selling the work - had a hope it was the real thing so he bought it and took it to Sotheby's for experts to assess.
Mr Fletcher said the painting was quite dirty at first - but, once it was cleaned, "this rather wonderful Rubens was revealed".
He added: "It's one of those moments that you have a couple of times a year when you walk in, and you just have this wonderful instant reaction of glee."
Some "hidden details" that helped convince him of the true artist were brought out with an infrared camera.
Mr Fletcher explained that examples of an artist making changes while painting can help prove its originality "as opposed to it being a slavish copy".
He added: "There is one enormous change in the sky where the red curtain descended... Rubens obviously wasn't happy with that so pushed it back up and included more sky.
"And to the contour of her dress, you can see that he painted it one way first and then... he's broadened her shoulders and the contour of her dress.
"And in a more minor way you can see lots of little changes in his painting of the fingers and hands."
He described the portrait, which depicts a young woman in a black dress and cloak, as "very majestic", adding: "She's got such a wonderful presence. I love the way she's looking out slightly out the corner of her eye with a little bit of a grin.
"There's a real sense of character to her face even though she's actually posed in quite a formal way."
The painting will be offered for auction at Sotheby's on 29 July, the first major evening sale in London since lockdown.