Experts have confirmed that the painting Rembrandt Laughing bought at an auction in Gloucestershire is an authentic self-portrait by the artist.
The anonymous buyer paid £2.2m for the painting which was valued at £1,500 in October at the Moore, Allen and Innocent auction house.
Specialists estimate the painting could be worth more than £15m as a self-portrait would be unique.
The picture is currently on loan to the Rembrandt House Museum in Amsterdam.
It is thought Rembrandt painted the self-portrait in approximately 1628 when he lived in his home town of Leiden.
The painting had been previously owned by an English family for more than 100 years.
Ernst van de Wetering, head of the Rembrandt Research Project, said: "It has an incredible presence. The light has the most natural quality of light you can think of."
Mr van de Wetering has published a 23-page analysis in which he explains why he believes Rembrandt was almost certainly the creator of the work.
He said: "Brush stroke, contour, materials and the monogram all point to the master's hand."
The picture measures 24.1cm by 16.5cm (9.5in by 6.5 in) and will remain in the Rembrandt House Museum in Amsterdam until 29 June.