A portrait long thought to be by a Rembrandt student has now been deemed to be by the 17th-century Dutch master himself, catapulting the value of the painting to about $30 million.

For 50 years, art historians relegated the portrait to "in the style of Rembrandt" until a recent re-evaluation by the Rembrandt Research Project.

Experts say more knowledge of the master's style fluctuations and his self-portraits has emerged in the last few decades, resulting in the new authentification. The new analysis was completed by leading Rembrandt scholar Ernst van de Wetering. Pigment and scientific testing will continue.

A gift to the British National Trust from the estate of an English aristocrat in 2010, the portrait hangs in the onetime home of Sir Francis Drake, Buckland Abbey, which serves as a national museum.

Signed "Rembrandt" and dated 1635, the painting depicts the artist in a big dark blue hat adorned with a white feather. He would have been 29 when it was painted.