Samson and Delilah, which depicts the Old Testament figure in the lap of the lover who betrayed him, was purchased at Christie’s in 1980 for £2.5 million, then the second-highest price ever paid for a painting at auction.
Some critics believe that the work, which only came to light in 1929, is a copy of a lost original that Rubens painted in about 1609.
Bought by German dealers from a Paris conservator, it was declared a Rubens by the now-discredited German scholar Ludwig Burchard, who, after his death in 1960, was found to have misattributed paintings for commercial gain.
Historian and author Euphrosyne Doxiadis has cast further doubt over the painting’s provenance by claiming that a former National Gallery trustee, Sir Isaiah Berlin, told her shortly before his death that he wanted “the truth” about Samson and Delilah to “come out”.
She said that Sir Isaiah, a British philosopher, had initially changed the subject when she criticised the painting at a dinner party in Athens in 1997.
But she claimed he approached her afterwards and said: “Write to [fellow trustee] Sir Keith Thomas… tell him from me that the truth will come out in the end – it always does – and the sooner it comes out the better it will be.”
When a fellow guest and historian drove Sir Isaiah to his hotel, he allegedly told her: “Your friend is right about the Rubens.”
Sir Isaiah died months later and Ms Doxiadis has tried in vain to contact Sir Keith. The Telegraph was also unable to reach him.
The claims have reignited the authenticity row.
‘In favour of truth’
Henry Hardy, Sir Isaiah’s editor and one of his literary trustees, said: “Berlin was certainly in favour of truth and against censorship.”
Hardy singled out a 1968 letter, in which Berlin wrote: “Everything must be published – small and great, black and white, embarrassing and ennobling.”
Michael Daley, director of ArtWatch UK, a museum watchdog, said: “Berlin made it clear that there was something to come out. Coming from someone like that, it’s serious. This will jolt them.”
Mr Daley, who has researched the painting extensively, said that “the National Gallery’s sums have never added up”.
“Its 1983 Technical Bulletin referred to its planed-down sliver of a panel, less than 3mm thick and glued onto a modern blockboard,” he added.
“But when exhibited in 1977 and sold in 1980, the picture had been described as an oak panel, with eyewitnesses recalling that it was bowed and reinforced on the back with crossed supporting bars.”
Planed ‘during the present century’
He also claims to have found Burchard’s 1930 certificate of authenticity in the picture’s dossiers, upgrading it to Rubens from Gerard van Honthorst, the Dutch artist.
“The Technical Bulletin claimed that the planing had occurred ‘probably during the present century’,” he said. “But Burchard said that the picture’s oak panel was in excellent condition and ‘even retained its original back’.”
Critics have argued that the colours are unsubtle and uncharacteristic of Rubens’s palette and question why the composition differs from two contemporary copies made from Rubens’s original.
But the National Gallery has refused to acknowledge artistic, technical or historical challenges.
‘Not transparent or responsive’
Ms Doxiadis told The Telegraph: “The National Gallery has certainly not been transparent or responsive in examining these long-standing and very serious questions about a publicly-owned painting in its care.”
Ms Doxiadis has published her latest research in UnHerd, the news and opinion website.
She argues that the picture was in fact painted by Gaston Lévi, a conservator who trained in Madrid: “That helps explain why the palette and brushwork in the contested work are so close to Spanish neo-impressionism, and so far from 17th-century Flanders.”
A National Gallery spokesman said: “Samson and Delilah has long been accepted by leading Rubens scholars as a masterpiece by Peter Paul Rubens.
“Painted on wood panel in oil shortly after his return to Antwerp in 1608 and demonstrating all that the artist had learned in Italy, it is a work of the highest aesthetic quality.
“A technical examination of the picture was presented in an article in the National Gallery’s Technical Bulletin in 1983. The findings remain valid.”