The Prince of Wales's architecture charity has been forced to explain its relationship with him following claims that it is acting as his "private lobby firm".
The Charity Commission raised a series of concerns with the Prince's Foundation for the Built Environment after receiving a complaint that the organisation had gone beyond its remit and tried to influence a number of planning decisions.
Republic, the anti-monarchy group, made the complaint to the charity regulator claiming Charles had used his organisation to enforce his "personal tastes".
Graham Smith, a spokesman for the organisation, welcomed the move and said: "The Charity Commission's reply to our complaint indicates there are serious questions to be answered about the relationship with Prince Charles and the charities he has set up and whether they are pursuing a public good, or working for Charles's benefit.
"We believe the relationship between him and the charities he has set up must be made clear, to ensure they're not being used as his own private lobby firms.
"Charities are legally obliged to remain independent and to take decisions in the interests of the objects of the charity. They are required to work for the public good, not personal interest."
Republic's complaint was based on claims made by the Guardian newspaper last month that Charles had used private pressure to influence the country's planning process.
Modernist designs for the Chelsea Barracks site in London were scrapped after the heir to the throne directly intervened by writing to the Middle East owners of the site and asking them to reconsider the proposals for the project.
When the plans were dropped in June, a spokesman for the development said Charles's Foundation would be amongst the stakeholders asked to discuss any new submissions.
Following the complaint, the Commission ordered the Foundation to explain its trustee decision-making, the activities it undertakes to further its charitable purposes for the public benefit, and its relationship with the prince.
A spokeswoman for the regulator said: "We have received a complaint about the charity, the Prince's Foundation.
"We have contacted the charity raising a number of issues relating to the complaint and concerns which have also been highlighted in the press, and have received a response from the charity.
"We will consider this response in assessing our regulatory role in the matter."