The BBC's Antiques Roadshow said it has uncovered a "hidden masterpiece" worth up to ?400,000.
The painting by the 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.
Father Jamie MacLeod 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 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.
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.
* Reuters