Painting by Rembrandt Harmenszoon van Rijn found at Buckland Abbey in Devon

It wasn't exactly a surprise find in an attic, but the National Trust's shock discovery that it has a £20 million Rembrandt hanging at one of its Westcountry properties is being regarded as very important news by the charity.

Not that the trust can or will sell the painting, which adorns the dining hall at Buckland Abbey – but its experts are delighted that the charity now has its first Rembrandt in a collection which includes over 13,500 works of art.

Curator of pictures and sculpture at Buckland Abbey David Taylor inspects the Rembrandt self-portrait
The recently confirmed self-portrait of Rembrandt discovered at Buckland Abbey

The Dutch Master's self-portrait was identified during a recent visit to the abbey by the world's leading Rembrandt expert, Ernst van de Wetering. For the past four decades the art-world had deemed it to be the work of one of Rembrandt's 17th century pupils.

The Dutch art historian, chairman of the Rembrandt Research Project, said the organisation's team last scrutinised the painting in 1968 when it decided the portrait was the work of one of the great man's students.

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"But, over the past 45 years we have gathered far more knowledge about Rembrandt's self-portraits and the fluctuations in his style," said Mr van de Wetering. "In 2005 I published an analysis of the genesis of the painting on the basis of an X-ray – this analysis and newly found circumstantial evidence remarkably increased the likelihood that the painting was by Rembrandt himself.

"But, to be sure I had to see the painting again for myself," he said. "My observations of the painting technique during my recent visit to Buckland Abbey were in tune with what I had observed with other paintings of that period among which the Belshazar in the London National Gallery and the (also erroneously rejected) so-called Rabbi in the British Royal Collection which show the same crude brushwork and painterly appearance."

Given to the charity in 2010, the painting now has an estimated nominal value of £20 million. But the trust has declared that – as it cares for items for public benefit – it will never be sold.

Instead, the work will be sent for specialist cleaning and further examination that will "shed more light on the painting's attribution" later this year.

The charity's curator of paintings and sculpture, David Taylor, said: "These latest investigations are incredibly exciting and important. The conservation work and technical analysis being undertaken over the winter will give us further confirmation regarding the picture's authorship."

Rembrandt Harmenszoon van Rijn (1606-1669) is widely regarded as one of the most prolific self-portraitists ever, with experts estimating that he painted 40-50 pictures of himself in oils and used himself as a model in 32 etchings and seven drawings.

The Buckland portrait shows Rembrandt aged 29, is signed Rembrandt and dated 1635.

The painting was given to the trust in September 2010 as a gift from the estate of the late Edna, Lady Samuel of Wych Cross. Her husband, the late Harold, Lord Samuel of Wych Cross, was a property developer and philanthropist who collected a great number of paintings, many of which are now on display at the Mansion House in London. This painting was previously owned by the Princes of Liechtenstein.

Jez McDermott, property manager at the abbey, said: "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. We never dared think that it might actually be an original and many of our visitors will have been just passed by it, in what is sure to be a real contrast to the attention it is now going to receive.

"Buckland's Rembrandt is certain to become one of the many highlights of the property for visitors to enjoy."

The painting will remain on display at Buckland for the next eight months before going away for cleaning and further investigation.