The Charity Commission is questioning Prince Charles's architecture charity following claims it has acted on his personal agenda by intervening in planning disputes.
Anti-monarchy campaign group Republic asked last month for an inquiry into the charity after a report in The Guardian claimed the prince had tried to persuade the developer responsible for a £500m office and shopping complex near St Paul's Cathedral to employ one of his preferred architects.
In a letter to Republic, seen by Third Sector, the regulator said it was questioning the Prince's Foundation for the Built Environment over "the charity's management and administration including trustee decision making, the activities it undertakes to further its charitable purposes for the public benefit and the charity's relationship with Prince Charles".
The commission letter said: "It appears that Prince Charles has expressed views in his personal capacity rather than as president of the charity.
"However, it is unclear to us whether the charity has intervened in the planning disputes or influenced the choice of architect. We do need to understand the charity's role in this."
Graham Smith, campaign manager at Republic, said: "We are concerned that the charities Charles has set up are parroting what Charles believes rather than acting in an independent manner.
"Charities are legally obliged to remain independent and to take decisions in the interests of their objects. They are required to work for the public good, not personal interest. We simply want to ensure this is the case with the foundation."
"We will continue to seek to hold him to account and demand more transparency over his attempts at political interference," he said.
Nobody at the Prince's Foundation for the Built Environment was available to comment.