Ludwig Burchard was one of the most respected scholars of the 20th century, revered for his knowledge of Rubens. But almost 50 years after his death, research suggests that he misattributed scores of paintings to the 17th-century master. On more than one occasion he may have given out certificates of authenticity for commercial gain.

Paintings to which he gave his blessing, and which made their way to collections worldwide, have since been demoted to lesser hands by the Corpus Rubenianum, the definitive study of Rubens by the Centre for Flemish Art in Antwerp.

Long-held doubts about the authenticity of Rubens’s Samson and Delilah are certain to be raised once more.

Research into Burchard — to be published in the latest Journal of ArtWatch UK, which campaigns for the welfare of works of art — has been conducted by Kasia Pisarek, a graduate of the Sorbonne in Paris who is completing a PhD at the University of Warsaw.

She began investigating him after being struck by some of the “truly improbable attributions” made by him, as well as other “well-known experts”.

She unearthed a document showing that in 1960 Burchard was accused by a French art dealer of being a dishonest man who had written a certificate for a painting that he knew was a copy. The downgraded Rubens works include Gideon Overcoming the Midianites, in the North Carolina Museum of Art.

Burchard identified it as the master’s work in 1954, addressing a certificate to a London dealer in which he noted “the vigour of the design, the brilliance of the vivid colours, the concentration of movement”.

Yet only four years earlier he had written to a fellow art historian to say the painting was missing the expected “transparency of shadows” and that it “seemed to me a compilation by a contemporary of Rubens”.

The Corpus Rubenianum has concluded that the painting is “by an unknown hand”, and the museum confirmed to The Times yesterday that the painting is now attributed to “a follower of Rubens”.

Ms Pisarek said: “We can see that Burchard came to the right conclusion in 1950, when giving informal advice to a colleague, but he changed his mind dramatically in 1954 in favour of an attribution to Rubens when formulating a certificate of authenticity for a commercial gallery.”

Burchard (1886-1960) came from Berlin to London in the mid-1930s, inspiring generations of academics with his scholarship. In 1996 the art critic Brian Sewell referred to him as “the great scholar of the day”.

Burchard’s greatest discoveries included the Samson and Delilah which for 180 years had been in the collection of the Princes of Liechtenstein. Inventories in 1767, 1780 and 1873 attributed it to a minor hand, Jan Van Den Hoecke, but in 1929 Burchard proclaimed it as a long-lost Rubens. On his recommendation a tobacco magnate purchased it from a gallery in Berlin. It was acquired by the National Gallery in 1980.

Although the National Gallery has insisted that it is just an “unusual” Rubens, the circle of scholars and artists who have doubted its authenticity welcomed the latest research. They argued that the picture is missing the wax seals that were applied to all paintings in the Liechtenstein collection in 1733 and that the number 18 in the bottom left-hand corner does not correspond to any known inventory number.

David Jaffe, senior curator at the National Gallery, said: “We’re confident the painting is by Rubens. It was shown very recently in the Gallery’s exhibition of Rubens’s work. It stood up very well against other works by Rubens.”

PAINTED OUT

Works downgraded since Burchard attributed them to Rubens include:

Albert, Archduke of Austria According to Hans Vlieghe in Corpus Rubenianum, the definitive study: “Much too hard and stereotyped” for Rubens

Helene Fourment as St Barbara Vlieghe: “A feeble, uninspired imitation”

St Bartholomew Corpus Rubenianum lists it as by “the studio of Rubens”

Jean de la Court Vlieghe: “A feebly painted 17th-century portrait”

Hendrik van Thulden Probably a studio work; was already doubted in the 18th century

Ferdinand, Cardinal Infante of Spain, on Horseback Burchard considered it a study for the Prado’s equestrian portrait. Vlieghe’s verdict: “No sign of Rubens’s quality. Must be a copy after the final version”

St Catherine of Siena Vlieghe: “The treatment seems to me inferior”

Head study of St Francis of Assisi Vlieghe: “I cannot accept the attribution.”

Research by Kasia Pisarek

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