The scandal surrounding Andrew Mountbatten-Windsor could end the reign of King Charles III, says a British historian who wrote an unofficial biography of the former prince.
Andrew Lownie, who spent years researching the royals for his book Entitled, says if the King had any idea of his brother's alleged misconduct while in public office, it could be cause for an early end to his reign.
"This is far more serious than the abdication crisis," Lownie told 7.30, referring to the abdication of King Edward VIII in 1936, when he chose to marry American divorcee Wallis Simpson.
"It could bring down King Charles.
"The BBC put a series of questions to Charles ... they refuse to answer quite sensible questions about what they knew.
"This is a real crisis, and they don't seem to have woken up to the seriousness of it."
After Mr Mountbatten-Windsor was arrested on suspicion of misconduct in public office, the king put out a statement supporting a proper investigation.
"I have learned with the deepest concern the news about Andrew Mountbatten-Windsor and suspicion of misconduct in public office," the statement read.
"What now follows is the full, fair and proper process by which this issue is investigated in the appropriate manner and by the appropriate authorities.
"In this, as I have said before, they have our full and wholehearted support and co-operation.
"Let me state clearly: the law must take its course."
Despite that statement of support for the investigation, Lownie believes that if the king knew of any of it, the crisis could deepen for the royals.
"If the royal family knew about that and didn't deal with Andrew, then the crisis is not just one about Andrew. It's one about the king himself," Lownie told 7.30.
"What people now want is his complete honesty about what he knew, when he knew it and what he did about it."
Authorities are investigating allegations that Mr Mountbatten-Windsor leaked confidential information while acting as trade envoy to the UK, a position he held between 2001 and 2011 after retiring from the navy.
The BBC had reported that those emails appear to show Mr Mountbatten-Windsor trying to promote controversial businessman David Rowland's financial ventures.
"We've had emails published saying that he was warned about Andrew and his association with a businessman called David Rowland," Lownie said.
"This was one of the people that Andrew was sharing confidential information with, and he [Charles] did nothing about it or appears to have done nothing."
'Two or three years of prison'
For all of the allegations about his close ties to Epstein, Mr Mountbatten-Windsor has never been arrested or charged over them.
The narrative against then-Prince Andrew started to shift in 2015, when he was named in US court documents by Virginia Giuffre, who accused Epstein of forcing her to have sex with the royal three times when she was 17.
In 2022, she filed a civil lawsuit against Mr Mountbatten-Windsor alleging sexual assault.
He settled the lawsuit but denied all allegations.
Public sentiment towards him shifted further last year, with the release of Giuffre's posthumous memoir, in which she claimed that she was trafficked to the former prince by Epstein.
"He was friendly enough, but still entitled — as if he believed having sex with me was his birthright," Giuffre wrote of Mr Mountbatten-Windsor.
His ties to Epstein have been revealed in several emails found in the Epstein files, and this time correspondence appears to show Mr Mountbatten-Windsor sharing reports on official trips he took to China, Hong Kong, Singapore and Vietnam.
After being interviewed by police, he was released without charge nearly 11 hours after his arrest, but remains under investigation.
ABC royal commentator Juliet Rieden described the arrest of the former prince as "a watershed moment in the history of the monarchy".
"We are in uncharted waters, completely uncharted waters. For a senior royal to be arrested, that has never happened in modern history," she said.
"I think it's incredibly damaging to the royal brand, and I think the monarchy is pedalling hard at the moment to try to rebuild that trust with the public."
As the monarchy seeks to rebuild trust, there is a question about whether Mr Mountbatten-Windsor could face jail time.
Jonathan Rogers, a criminal justice expert at Cambridge University, doesn't believe that Mr Mountbatten-Windsor being found guilty would result in a lengthy prison sentence, despite serious charges.
"The maximum sentence is lifetime imprisonment. I don't think people should get too excited about the prospect of that happening," Mr Rogers said.
"I would imagine that somebody convicted of comparable offences would normally expect no more than two or three years of prison."
As to the likelihood of the former Duke of York being convicted if charged with misconduct in public office, Dr Rogers said the prosecution would have to prove four things.
First, that the defendant held a public office. Secondly, that he wilfully neglected or abused his position. Third, that his neglect or abuse was so bad as to amount to an abuse of the public's trust and fourth, that there would be no particular defence for his actions as I see it.
"If they could prove the first three, there would not be any defence open to Andrew."
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