A painting bought for £400 that was revealed to be a Van Dyck portrait during an episode of Antiques Roadshow is expected to fetch up to £500,000 when it goes under the hammer tonight.
The work was bought 12 years ago by Father Jamie MacLeod from an antiques shop in Cheshire and was identified after the show's host, Fiona Bruce, saw it and thought it might be genuine.
Ms Bruce, who was making a show about the artist with expert Philip Mould, asked him to look at it and after a lengthy restoration process the painting was verified by Dr Christopher Brown who is one of the world's authorities on Van Dyck.
Father MacLeod, an Old Catholic priest who runs a retreat in the Peak District, said: "It has been a blessing to own this magnificent portrait which has given me great pleasure over the years.
"I will be sad to part with it, though the proceeds will be put to excellent use, going towards the acquisition of new church bells for Whaley Hall Ecumenical Retreat House in Derbyshire to commemorate the centenary of the end of the First World in 2018."
The painting, which has an estimated price of £300,000 to £500,000, is believed to be a sketch for a work called the Magistrates Of Brussels which hung in the city's Town Hall until it was destroyed by a French attack in 1695.
"The picture is of great importance as it provides a fascinating insight into van Dyck's working method and also constitutes a significant surviving document for the artist's lost group portrait of The Magistrates of Brussels."
The painting has been public view in London and at Christie's in New York ahead of the sale. It will be sold at the auction house's London sale of Old Master and British Paintings.
The Antiques Roadshow episode featuring the work was broadcast on BBC One in December.
Sir Anthony Van Dyck was born in modern-day Belgium and came to work in England in 1632 at the invitation of King Charles I.