A painting donated to the National Trust by the estate of a wealthy supporter has been identified as a previously unknown famous Dutch painter Rembrandt self-portrait worth £20m.

The painting was given to the trust in 2010 by the estate of Edna, Lady Samuel of Wych Cross, whose property developer husband was a major collector of Dutch and Flemish art.

The portrait of the artist wearing a cap with a large white feather was previously thought to have been done by one of Rembrandt’s pupils and was credited as being in the “style of” the 17th century Dutch master.

But the National Trust said an investigation led by Ernst van de Wetering, the world’s leading Rembrandt expert, determined it was by the master himself.

“It’s amazing to think we might’ve had an actual Rembrandt hanging here on the walls at Buckland Abbey for the past couple of years,” said Jez McDermott, the property manager at the house, the former home of 16th century seafarer Sir Francis Drake.

“We never dared think it might actually be an original.”

Mr van de Wetering, the chair of the Rembrandt Research Project, said researchers in 1968 had concluded the painting – which features Rembrandt’s signature and is dated 1635, when the artist was aged 29 – was likely to have been done by a pupil.

“But over the past 45 years we have gathered far more knowledge about Rembrandt’s self-portraits and the fluctuations in his style,” he said.

He said the painting’s crude brushwork and painting technique can be observed in other paintings from the early stage of Rembrandt’s career.

Although the painting has been valued at £20m, the trust, whose mandate is to safeguard Britain’s heritage, is not allowed to sell it.

It said the painting would hang at Buckland Abbey for eight more months before being sent for cleaning and further examination, including X-rays and infrared testing.

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