Republic has lodged a formal complaint with the Metropolitan Police over its handling of cash for honours allegations, accusing the force of an “unacceptable” lack of action.

In September, Graham Smith, chief executive of the anti-monarchy group, reported both the Prince of Wales and his former most trusted aide Michael Fawcett to Scotland Yard on suspicion of breaching the Honours (Prevention of Abuses) Act 1925.

Ex-royal valet Mr Fawcett is accused of promising to help secure a knighthood and British citizenship for a Saudi billionaire donor.Mr Fawcett stepped down as chief executive of Charles’ charitable organisation The Prince’s Foundation after a string of allegations.

Mr Smith said: “The Metropolitan Police seem reluctant to investigate those in power, the royals in particular.

“They claim to work without fear or favour but that doesn’t appear to be the case.”

He added: “We saw this with the Virginia Roberts case, when she reported Prince Andrew for (alleged) sex offences that were said to have been committed in London. The Met showed no interest and took no action.

“Now the police seem determined to forget the cash-for-honours case in the hope that it will go away.”

Mr Smith described the situation as “unacceptable” and in his complaint said: “There is a clear public interest in investigating the integrity of the honours system and the office of head of state and heir.

“So it’s unclear why there has been no obvious progress on this matter.”

In December, an investigation ordered by the Foundation and carried out independently by auditing firm Ernst & Young found Mr Fawcett co-ordinated with “fixers” over honours nominations.

The Duke of York
The Duke of York denies the allegations against him (PA)

A Clarence House spokesman has said: “The Prince of Wales has no knowledge of the alleged offer of honours or British citizenship on the basis of donation to his charities.”

Scotland Yard has yet to comment.

Charles’s brother The Duke of York is facing a US civil sexual assault trial and denies the allegations.

The independent investigation into fundraising practices was ordered by the foundation and carried out independently by auditing firm Ernst & Young.