PARIS (AFP) - A French documentary has cast fresh doubts over the world's most expensive painting, the Salvator Mundi, credited to Leonardo da Vinci, revealing a resulting diplomatic tussle between France and its Saudi owner.

The painting of Jesus Christ, nicknamed the "male Mona Lisa", was sold at a 2017 Christies auction in New York for a record US$450 million (S$604 million).

Its secret buyer was later revealed to be Saudi Crown Prince Mohammed bin Salman, though this is still denied in Riyadh.

But there have long been questions over whether it was entirely the work of da Vinci.

The issue resurfaced when the painting failed to appear as planned at the Louvre Abu Dhabi museum in 2018, and then at a blockbuster da Vinci show by the Louvre in Paris the following year.

Now a documentary, The Savior For Sale by film-maker Antoine Vitkine, to be premiered on French TV next week, sets out to reveal what was going on behind the scenes.

In the film, senior officials from President Emmanuel Macron's government, appearing under pseudonyms, confirm that the Louvre's scientific analysis of the painting concluded that while it was produced in da Vinci's workshop, the master himself only "contributed" to the painting.

This apparently went down badly with the Saudis.

Row over authenticity

"Things turned incomprehensible," says one of the French officials in the film. "The request by 'MBS' (Prince Mohammed) was very clear: Show the Salvator Mundi next to the Mona Lisa, and present it as 100 per cent a da Vinci."

The Saudis offered various deals, the official says, but his recommendation to the Elysee was that this would amount to "laundering a US$450 million artwork".

The documentary alleges that some members of the French government, including Foreign Minister Jean-Yves Le Drian, lobbied on behalf of Prince Mohammed's request.

They were concerned about the impact on France's wide-ranging strategic and economic relationship with Saudi Arabia.

But Mr Macron ultimately decided to reject Prince Mohammed's request, leaving it to the Louvre to negotiate with the Saudis on how the painting should be presented in their retrospective, said the documentary.

No deal was concluded and the museum has refused to comment on the case.

"The Saudis are afraid of this debate on the authenticity," says Mr Chris Dercon, who heads one of France's top museum bodies and advises the Saudi government on art, in the documentary.

"They are afraid that people will say, both at home and abroad, 'You spent all this money for something that is not a da Vinci.'"

The painting was initially bought in 2005 for just US$1,175 by a New York art dealer and restored in the US.

Several British experts authenticated the painting as a long-lost da Vinci and it was presented as such at London's National Gallery in 2011 before being sold to a Russian oligarch for US$127.5 million two years later.