Prince Charles gave an honour to Lord David Brownlow, after the Tory peer spent £1.7m to bail out the royal’s eco-village project in Scotland.
The Prince of Wales also opened up his 18th-century country house, Dumfries House, for Brownlow’s 50th birthday and awarded his company a £1.2m construction contract.
The Sunday Times reports that the Scottish Charity Regulator is now investigating the decision to make Brownlow a Commander of the Victorian Order (CVO).
He was also made a trustee of the Prince’s Foundation, which came after Brownlow’s firm bought 11 properties in the Knockroon eco-village project.
Brownlow is most well known for paying for the refurbishment of Boris Johnson’s Downing Street flat last year.
The royal is already being investigated by the Metropolitan Police for his role in securing a CBE for Saudi billionaire Mahfouz Marei Mubarak bin Mahfouz, after he gave £1.5m to Prince Charles’ causes.
It comes after it was last week revealed that the Prince of Wales had accepted millions of pounds in suitcases from a top Qatari politician as a donation for his charity.
A Prince’s Foundation spokeswoman said: “Lord Brownlow was appointed CVO in 2018 in recognition of his role of chair of the charity The Prince’s Foundation for Building Community.”