A leading old masters specialist has surprised the art world by identifying three previously overlooked paintings by the 17th-century artist Anthony van Dyck.

Philip Mould, a British dealer who once bought a Gainsborough on eBay for £120, has proved his eagle eye once again with the find, which includes two paintings sold by Christie's last year as anonymous works. They were estimated to fetch just £20,000 and £8,000 respectively. The third, dismissed as a 19th-century copy, had been relegated to a storeroom of the Bowes Museum in Co Durham.

Van Dyck (1599-1641) was the most revered painter in 17th-century Britain, where he was court artist to Charles I. The latest discoveries by Mould, who appears on BBC1's Antiques Roadshow and has a record of unearthing masterpieces, have been corroborated by other Van Dyck academics. Asked how one of the world's leading auction houses could have overlooked two works by the Flemish master, Mould told the Observer: "Perhaps Christie's didn't have sufficient time to research these pictures."

Although he acknowledged that auctioneers do not have the benefit of cleaning and restoring works, which help to reveal true quality, he added: "As art dealers, we scour daily the world's auction catalogues for paintings that are … wrongly identified … In any week, our finds might range from a misidentified Tudor icon to a misattributed 18th-century landscape … but by a strange chance we seem to have hit a seam of Van Dycks."

One of the paintings surfaced among artworks that were looted by the Nazis and were auctioned last September by Christie's in Paris following their return to their original owners, the Rothschilds. Leafing through the catalogue, Mould was struck by a "devastatingly pretty portrait of a girl".

It was described by Christie's merely as "Flemish School, 17th century, portrait of a young girl with a fan", discounting its own record that it was once attributed to Van Dyck. The presale estimate of €15,000 to €20,000 (£13,200-£17,600) was a fraction of its value as a Van Dyck.

When they saw the photograph, Mould and his colleague Bendor Grosvenor were "90% sure" it was by Van Dyck. Despite layers of dirt, it was "still possible to see that we were dealing with a painting of potentially the very highest quality", said Mould. "The sitter's expression alone tells us that this is not a portrait of a miniature adult, as one so often sees in the 17th century, but of a child. Few artists were then capable of such a subtle characterisation – and perhaps the best was Van Dyck."

Such was their excitement that they dared not hover over it at the sale preview. "You have to look casually and pretend to be interested in a few other duds," Grosvenor said.

Within days, the pair's detective work had unearthed crucial documentary evidence, including a 200-year-old scholarly study and a 1940s photograph taken by the Nazis which described the painting as a Van Dyck. They also tracked down previous owners, including John Smith, a noted 19th-century connoisseur-dealer, who had detailed the picture, down to the colour of the dress, and noted its sale by Christie's in 1835 for 169 guineas. Mould unearthed the 1835 sale catalogue that described it as a Van Dyck.

All this information gave Mould the confidence to bid way beyond the estimate, although, he admitted, "there is a note of fear that runs through all of us – which is how badly you can screw up if you get it wrong". As bidding soared to €1m, it became clear that another buyer had recognised the painting's potential. But Mould won. Days later, his restorer removed dirt and varnish, revealing subtler brush strokes and colours, especially around the eyes. The child's hands, which in the catalogue photograph lacked the elegance of Van Dyck's elongated fingers, emerged as sensitively rendered. Now, as a rare Van Dyck child portrait, it is valued at £3.5m.

Mould's second painting had been relegated to Christie's secondary London saleroom in South Kensington, where the catalogue described it as "Circle of Van Dyck – Head Study of an Old Man", giving a presale estimate of £5,000 to £8,000. Mould's research identified it as the head of St Joseph for a lost painting of the holy family, known from a version by the master and his studio assistants in the Manchester Museum. Like the first painting, it was covered in dirt and discoloured varnish.

"Nevertheless," Mould said, "we were struck by its quality as soon as we saw it in the catalogue." He bought it for £121,250. Conservation confirmed his suspicions. As a Van Dyck oil study, it is worth £350,000.

The third picture to be discovered over this 18-month period came to Mould's attention through the Public Catalogue Foundation's work in photographing all oil paintings in British public collections. His eye was caught by a picture catalogued as "After Van Dyck, Portrait of Olivia Porter".

It had suffered from clouded varnish and some botched restoration, but the dealer was struck by the sophisticated drapery, colouring and facial expression – all typical of Van Dyck's female portraits of the 1630s, the artist's period in England.

The painting will be displayed in its unrefined state in a major loan exhibition, Finding Van Dyck, at Philip Mould Fine Paintings in London. Mould, who will present a BBC1 series, Fake or Fortune, from June 19, plans to reveal some of his art sleuth secrets, such as how to "read" through dirt and discoloured varnish, and showing "the risks, the science and the processes" involved in searching for unidentified old masters.

Various collectors and national institutions are lending paintings for the exhibition and these will sit alongside other masterpieces, including Van Dyck's last self-portrait and a Titian.

A Christie's spokesman said: "Attributions can and do change over time. The possible attribution to Van Dyck was considered for both of these works. They were shown to internationally renowned experts and museum curators before they were offered at auction. The public auction saw both works attract determined bidding nonetheless and both works sold for full prices."

• The following correction was printed in the Observer's For the record column, Sunday 19 June 2011. The Holy Family With the Head of St Joseph is in the collection of Manchester City Galleries, not Manchester Museum ("Trail of clues that led to Van Dyck's lost masterpieces", News).