The Metropolitan Police have launched an investigation into an alleged cash-for-honours scandal after the Prince of Wales and his former close confidant, Michael Fawcett, were reported over the claims.

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.

Now there is the prospect of further damage with the probe into claims that a donor to Charles’s charitable body, The Prince’s Foundation, was offered help in securing a knighthood.

Pressure group Republic contacted Scotland Yard last September and reported both the future king and Mr Fawcett, 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 Mr Fawcett.

At the time Clarence House said the prince had “no knowledge” of the alleged cash-for-honours scandal.

Graham Smith, chief executive of Republic, which is anti-monarchy, said: “I think we want to know that Prince Charles himself will be investigated along with Michael Fawcett.

“So we hope the investigation will be carried out without fear or favour and will as be thorough as it needs to be.”

Mr Fawcett, who has since resigned as chief executive of The Prince’s Foundation in the wake of the scandal, was accused of promising to help secure a knighthood and British citizenship for a Saudi billionaire donor.

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.

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 reported to have donated large sums to restoration projects of particular interest to 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 in a statement: “The Metropolitan Police Service has launched an investigation into allegations of offences under the Honours (Prevention of Abuses) Act 1925.

“The decision follows an assessment of a September 2021 letter. This related to media reporting alleging offers of help were made to secure honours and citizenship for a Saudi national.

“The Special Enquiry Team has conducted the assessment process which has included contacting those believed to hold relevant information.

“Officers liaised with The Prince’s Foundation about the findings of an independent investigation into fundraising practices. The foundation provided a number of relevant documents.

“These documents were reviewed alongside existing information. The assessment determined an investigation will commence.

“There have been no arrests or interviews under caution.”

Charles is president of the foundation but not involved with its governance, with the charity’s trustees overseeing its day-to-day activities.

Mark Stephens, an international reputation lawyer from the law firm Howard Kennedy, said he thinks it highly unlikely that the prince will be interviewed by police about the allegations.

He said: “Charles has an entire staff who runs his office and he’s only told about the intimate dealings when he needs to know about them, and he clearly wouldn’t have needed to know about this.”

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.”