A portrait that had gathered dust in a family collection for almost 140 years could make £3.5 million ($4.3 million) at auction after it was discovered to be a Rubens.

The origins of "Portrait Of A Lady" were revealed when layers of dirt and varnish were removed, and it will go under the hammer at Sotheby’s in London, with a £2.5 million to £3.5 million ($3.1-4.3 million) estimate, Belfast Telegraph reports.

The 17th century masterpiece was exhibited at the Royal Academy in London in 1902 as a work by Rubens, but Andrew Fletcher, Sotheby’s head of department for Old Master paintings, said its origins had been “forgotten about”.

After remaining in one family collection for 139 years, it was sold for £78,000 in 2017.

It was catalogued during the sale as being from the workshop of Rubens, meaning it was thought to have been painted by one of the his assistants, possibly overseen by the Old Master himself.

An anonymous buyer – who is now selling the work – had a hope it was the real thing so he snapped up the painting and took it to Sotheby’s for experts to take a look.

“It was quite dirty, with hundred years of dirt and old varnish on it” but when it was cleaned “this rather wonderful Rubens was revealed”, Mr Fletcher said.

“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,” he said.

Some “hidden details” were brought out with an infrared camera.