The Prince of Wales and his most trusted aide Michael Fawcett have been reported to the police by the pressure group Republic over the cash-for-honours allegations.
Graham Smith, chief executive of the organisation which campaigns for an elected head of state, said: “Failure to properly investigate these matters will damage public trust in the police, the royals and the honours system.”
Mr Smith said he had contacted the Metropolitan Police and reported both Charles and Mr Fawcett on suspicion of breaching the Honours (Prevention of Abuses) Act 1925.
Watch: Prince Charles’ former aide keeps silent over honour claims
Former royal valet Mr Fawcett is accused of promising to help secure a knighthood and British citizenship for a Saudi billionaire donor.
Mr Smith said: “Time and again the royals blame their staff or associates for their own mistakes.
“It is difficult to believe Charles wasn’t aware of these arrangements or promises. It’s time the royals were personally challenged over their conduct.”
Ex-Liberal Democrat MP Norman Baker has also written to the Met asking the force’s Commissioner, Dame Cressida Dick, to launch a criminal probe.
Mr Baker, the author of a book about royal family finances entitled And What Do You Do? said: “There appears to be prima facie evidence that an offence has been committed under the 1925 Honours (Prevention of Abuses) Act and it needs to be investigated.”
He called on Charles to make a statement, saying: “The prince should own up and accept responsibility for this.
“The history of him and Michael Fawcett is that when things go pear-shaped, Charles disappears into the background and pretends he doesn’t know the bloke and Michael falls on his sword, and then when a decent period of time has elapsed he gets brought back in when no-one is looking.”
A spokesman for Scotland Yard said: “We are aware of the media reports and await further contact in relation to this matter.”
Mr Fawcett has stepped down temporarily from his role as chief executive of Charles’s The Prince’s Foundation, while the charity investigates his actions.
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 KBE, and support his application for citizenship.
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, who is listed as a supporter on The Prince’s Foundation website, is said to have donated large sums to restoration projects of particular interest to Charles. He is said to deny any wrongdoing himself.
Mr Fawcett was Charles’s most indispensable aide, with the prince once declaring: “I can manage without just about anyone, except for Michael.”
It fell to Mr Fawcett to squeeze out the future king’s toothpaste when he broke his arm playing polo.
He quit as Charles’s personal assistant in 2003 after being dubbed “Fawcett the Fence” and accused of selling royal gifts.
He faced allegations of selling unwanted royal gifts and pocketing a percentage of the proceeds, but was cleared by an internal inquiry of any financial misconduct.
The Peat Inquiry, headed by Charles’ then private secretary Sir Michael Peat, found Mr Fawcett did “infringe internal rules relating to gifts from suppliers” but could not be severely criticised because the rules were not enforced and he made no secret of such gifts.
Yet the report painted a picture of Mr Fawcett as an alleged bully who enjoyed lavish perks and privileges and accepted valuable gifts from outsiders.
He resigned with an undisclosed cash severance package and an agreement to work as the prince’s freelance events manager, and went on to set up his party planning company Premier Mode.
He organised the private party for the Queen’s 80th birthday, hosted by Charles, and took charge of Charles and Camilla’s engagement celebration dinner at Windsor in 2005.
After five years as chief executive of Dumfries House Trust, he was appointed to the major role of chief executive of Charles’s The Prince’s Foundation in 2018 amid a reorganisation of the prince’s charities.
A Clarence House spokesman said: “It is right that The Prince’s Foundation is taking the investigation so seriously and we will not be commenting further whilst it is ongoing.”
Watch: Charles’s ex-aide steps down from charity role amid allegations