Auction house Christie's has insisted that Leonardo Da Vinci was the true creator of the world's most expensive painting Salvator Mundi, amid doubts about its origins.
The portrait of Jesus was sold for $450million in New York and acquired by the Saudi royal family in 2017.
But experts have warned of 'telling details', including a poorly-drawn finger, suggesting it was made by Da Vinci's workshop and not by the painter himself.
The row has intensified as Italy and France prepare to mark the 500th anniversary of Da Vinci's death on Thursday.
A Christie's spokesman said: 'We stand by the thorough research and scholarship that led to the attribution of this painting in 2010.
'No new discussion or speculation since the 2017 sale at Christie's has caused us to revisit its position.'
But many art experts remain unconvinced of the painting's authenticity.
'Certain details are very telling,' said Jacques Franck, a specialist in Da Vinci's technique.
Highlighting the painting's poor depiction of a finger, he said some elements of the painting were 'anatomically impossible'.
At the time the canvas was painted in around 1500, Da Vinci had his workshop complete certain paintings because he himself had very little time, he said.
Daniel Salvatore Schiffer, another Da Vinci expert, also believes the painting was not done by the Italian master.
'When you analyse the details, nothing is by Leonardo, it doesn't have his spirit.'
Ben Lewis, an art historian who wrote 'The Last Leonardo' said London's National Gallery, which exhibited the painting in 2011, had not taken on board the advice of five experts who were sent to authenticate the painting.
Although two of them believed it was authentic, another didn't, and the others were unsure. But the painting was presented at the exhibition as a genuine work by Leonardo da Vinci.
But Diane Modestini, who worked on the restoration of the painting from 2005, said she did not understand the controversy, insisting that 'Leonardo da Vinci painted it'.
The painting has not been displayed in public since the auction in New York in November 2017.
It was due to go on display at the Louvre Abu Dhabi in September last year, but its unveiling was postponed without explanation.
Abu Dhabi's Department of Culture and Tourism apparently 'acquired' the painting after it was purchased by the UAE's close ally, Saudi Arabia.
The buyer was reported to be Saudi prince Badr ben Abdallah, acting in the name of powerful crown prince Mohammed bin Salman.
Prince Badr was appointed to head the kingdom's culture ministry in a government shakeup in June.
According to information service Artprice, a public display has been blocked on religious grounds by clerics at Al-Azhar university in Cairo.
Jesus is seen as a prophet in Islam, which prohibits any physical depiction of God. But the picture portrays him as a saviour and thus a deity.
The Louvre in Paris has asked Abu Dhabi for the painting to be given on loan ahead of a Da Vinci exhibition later this year, but has received no response.
The French and Italian Presidents will travel to the Loire Valley on Thursday to mark Da Vinci's death there in 1519.