A British television show dedicated to valuing people's usually modest antiques said that it has uncovered a "hidden masterpiece" worth up to $807,000.
The painting by 17th-century Flemish artist Anthony van Dyck depicts a bearded Brussels magistrate wearing a ruff and was brought to the show by an English priest who bought it in an antiques shop for only 400 pounds.
Father Jamie MacLeod, who purchased the painting because he liked the thick gold-coloured frame, plans to sell the portrait to fund the restoration of bells at the chapel of a religious retreat he runs in Derbyshire, England.
Philip Mould, an art expert working for the BBC's Antiques Roadshow, had suspected that the painting might be an original Van Dyck and had urged the cleric to have the canvas stripped back to its original paintwork and authenticated.
Christopher Brown, one of the world's leading authorities on Van Dyck and director of the Ashmolean Museum in Oxford, was then able to verify that the painting was genuine, the programme said.
The portrait is believed to have been completed as part of Van Dyck's preparation for a larger 1634 work showing seven magistrates. That painting has since been destroyed.
Mould described the find as "a thrilling example" of the skills of direct observation that made Van Dyck such a great portrait painter.
Van Dyck was one of England's leading court painters in the 17th century, making his name with portraits of Charles I of England and his family and court.
Fiona Bruce, a presenter on the BBC show, said she had suspected the canvas was a Van Dyck when she first saw it.
"It's everyone's dream to spot a hidden masterpiece. To discover a genuine Van Dyck is incredibly exciting," she said.