Testing has confirmed that a painting donated to England's National Trust is indeed a 400-year-old Rembrandt self-portrait.

The painting, which is worth an estimated $50 million, was tested by the Hamilton Kerr Institute over an eight-month period and then examined again by the foremost Rembrandt expert. Both came to the conclusion that the self-portrait was done by Rembrandt in 1635.

The painting, which currently hangs in Buckland Abbey, was donated in 2010 to the Trust. However, since 1968, the work has been dogged by claims it wasn't the genuine thing. The Rembrandt Research Project and Horst Gerson were doubtful of the self-portrait due painting and panel oddities.

Then in 2005, expert Ernst van de Wetering believed that it might actually be a Rembrandt after all and seven years later insisted it was true after further study. So, thanks to funds raised by Players of People's Postcode Lottery, further examination and cleaning on the painting was conducted.

HKI paintings conservator Christine Slottvedd Kimbriel said that upon receipt of the portrait, the institute began putting through several different analyses, including infra-red reflectography, raking light photography, pigment and medium analysis and x-radiography.

"Careful cleaning and removal of several layers of aged and yellowed varnish which had been added to the painting much later, revealed the original colours and painting style beneath," Kimbriel said. "What was revealed was a true depth of colour, much more detail and a three-dimensional appearance to the fabric."

The HKI also examined the signature, which had also been called into question by doubters of the self-portrait's authenticity and it was found through a cross-section analysis that it was done at the time of the work's initial completion.