A filthy oil painting locked away in a museum in the northeast of England was on Saturday revealed to be an original masterpiece by Van Dyck.

The portrait was spotted when it was photographed for an ambitious project to catalogue every single one of Britain’s oil paintings in public ownership in an online museum.

Depicting Olivia Boteler Porter, lady-in-waiting to Henrietta Maria, the wife of English king Charles I, the 17th century painting had been listed as “a copy after Sir Anthony Van Dyck”.

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But when experts took a closer look, they realised that the oval portrait, housed in The Bowes Museum in Barnard Castle in County Durham, was an original.

“To find a portrait by Van Dyck is rare enough, but to find one of his ‘friendship portraits’ like this, of the wife of his best friend in England (Endymion Porter), is extraordinarily lucky,” said Bendor Grosvenor, an art historian and dealer.

He said the painting had been in such a bad state that it would have likely only fetched up to £5,000 ($7,500, 5,700 euros) at auction as a Van Dyck copy. But now, it could be valued at up to £1 million.

The online museum, Your Paintings, is a 10-year project cataloguing 210,000 paintings from across Britain. It was organised by the Public Catalogue Foundation and the BBC.