The portrait was previously thought to have been a copy and was in storage at the Bowes Museum in County Durham.
But it was photographed for a project to put all of the UK’s oil paintings on the BBC Your Paintings website, where it was seen by an art historian.
After an investigation by BBC Two’s The Culture Show, it has been verified by Van Dyck expert Dr Christopher Brown.
Dr Brown, director of the Ashmolean Museum in Oxford, told The Culture Show that it was “a substantial discovery”.
“It’s quite clear that this is a late English period Van Dyck,” he said. “I don’t think there’s any doubt about that. This absolutely is Van Dyck at his best.”
Anthony Van Dyck, one of the finest portrait painters of the 17th century, was born in Antwerp and became court artist for King Charles I in London in 1632. The newly identified portrait depicts Olivia Boteler Porter, lady-in-waiting to Charles I’s wife Henrietta Maria.
The painting, which was not thought to be important, was covered in dirt and varnish and was not on display at the museum.
It was spotted by art historian and dealer Dr Bendor Grosvenor. “For insurance purposes it should now be valued at anything up to £1m,” he said: “Had it appeared at auction as a copy, and in its dirty state, it would probably only have been estimated at about £3,000-5,000.”