The Prince of Wales would be "happy" to help a new UK Metropolitan Police investigation into an alleged cash-for-honours scandal launched after he and a former close confidant were reported to officers over the claims.
Anti-monarchy group Republic made a formal complaint to Met detectives about the heir to the throne and Michael Fawcett last September, following a series of newspaper articles alleging a donor to The Prince's Foundation was offered help securing a knighthood.
At the time Clarence House said the prince had "no knowledge" of the alleged cash-for-honours scandal.
On Wednesday, the Met announced there was just cause to launch an investigation into the affair.
Mr Fawcett, who has since resigned as chief executive of The Prince's Foundation in the wake of the alleged scandal, had been accused of promising to help Saudi billionaire donor Mahfouz Marei Mubarak bin Mahfouz achieve British citizenship and a knighthood.
A source said about the police investigation: "His Royal Highness is happy to help if asked. He has not been."
The royal family has already seen its reputation bruised by the Duke of York's sexual assault civil case, which was settled out of court this week, and now there is the prospect of further damage with the launch of the Met Police probe.
Pressure group Republic contacted Scotland Yard and reported both the future king and Mr Fawcett, Prince Charles's former valet, on suspicion of breaching the Honours (Prevention of Abuses) Act 1925.
Ex-Liberal Democrat MP Norman Baker also wrote to the Met asking the force to launch a criminal probe into the allegations made against the former royal confidant.
The police investigation comes just days after Dame Cressida Dick - who had said they had no plans to investigate the matter - resigned from her role as commissioner.
Last autumn, the Mail on Sunday published a letter from 2017 in which Mr Fawcett reportedly wrote that he was willing to make an application to change businessman Mahfouz Marei Mubarak bin Mahfouz's honorary CBE to a knighthood, and support his application for citizenship.
From left to right: Michael Fawcett, Camilla, Duchess of Cornwall, and Prince Charles, Prince of Wales, pictured in 2108. Source: Getty Images
The letter, written on headed notepaper in Mr Fawcett's then capacity as chief executive of the Dumfries House Trust, said the applications would be made in response to "the most recent and anticipated support" of the trust.
Mr Mahfouz, is reported to have donated large sums to restoration projects of particular interest to Prince Charles. He is said to deny any wrongdoing himself.
The Prince's Foundation commissioned an independent investigation into the allegations, which found evidence of Mr Fawcett's "communications and co-ordination" with "so-called 'fixers' regarding honorary nominations for a donor between 2014-18".
Scotland Yard said police had reviewed the independent investigation into fundraising practices and "determined an investigation will commence".
There have been no arrests or interviews under caution, it added.
Prince Charles is president of the foundation but not involved with its governance, with the charity's trustees overseeing its day-to-day activities.
Clarence House reiterated its previous statement: "The Prince of Wales had no knowledge of the alleged offer of honours or British citizenship on the basis of donation to his charities."
