The painting was bought 12 years ago by Father Jamie MacLeod from an antiques shop in Cheshire for just £400.
The sketch was largely obscured by later overpaint but recent cleaning and restoration - followed in a special BBC show broadcast last December - revealed a swiftly executed head study consistent with three other known sketches for the Magistrates of Brussels, van Dyck's full-length group portrait painted in 1634-35.
The picture hung in the Brussels Town Hall until it was destroyed during the French bombardment of the city in 1695. The proceeds of the sale will go towards the acquisition of new church bells for Whaley Hall Ecumenical Retreat House in Derbyshire to commemorate the centenary of the start of the First World War.
