Fawcett, who previously served as the Prince's valet, had voluntarily vacated the post of chief executive in early September following accusations he used his position and influence to help secure an honorary title and British citizenship for a Saudi businessman in exchange for donations.
The Prince's Foundation launched an inquiry into the claims. Charles has denied any knowledge of the alleged offer of honors in return for donations, with a Clarence House spokesperson previously telling CNN the prince fully supported the charity's ongoing investigation.
A friend of Fawcett's told Britain's Daily Mail newspaper, which first reported the resignation: "Michael has resigned and he will never be coming back. He has lost five stone in weight and is a shadow of his former self."
Fawcett has been accused of using his position and influence to help Saudi businessman Mahfouz Marei Mubarak bin Mahfouz obtain an honorary Commander of the Most Excellent Order of the British Empire (or CBE) title, according to the Sunday Times.
An honorary CBE is one of the highest accolades that can be received by a non-Commonwealth citizen. The Sunday Times reported Mahfouz made large financial donations to refurbishment projects of interest to the Prince of Wales in exchange for support in securing the honorary title, which the Queen conferred in 2016. The newspaper alleges Fawcett -- who left his valet role in 2003 but was reportedly freelancing for the royal -- coordinated the application process.
The Sunday Times reported Mahfouz -- who denies any wrongdoing -- was seeking numerous honorary roles to bolster his application for British citizenship through the "golden visa" for investment program.
CNN has reached out to Mahfouz for comment. CNN also tried to reach Fawcett through the Prince's Foundation.
The Prince's Foundation is an umbrella organization for a number of Charles' charitable projects, based at the historic Dumfries House estate in Scotland, which is used as a center for education and training and champions sustainable living. Fawcett had been chief executive of the charity since 2018, according to PA Media news agency. It is operated separately from Prince Charles' main office at Clarence House.
Fawcett, whom the Prince of Wales once described as "indispensable," joined the royal service in 1981 as a footman to the Queen. He earned quick promotion to sergeant footman before being made assistant valet to Charles.
The royal aide resigned from Charles' service in 2003 following allegations of mismanagement of his royal household. A report later cleared Fawcett of selling unwanted royal gifts and taking a cut of the profit. However, he was found to have bent rules and accepted perks and hospitality.