The Royal Institute of British Architects (RIBA) has accused Prince Charles of using his position to exert "undue influence" to sway planning decisions.
The first woman president of the RIBA Ruth Reed said that correspondence about proposed developments behind the scenes would be "an abuse of his position."
She told BBC Radio Four's Front Row: "It is unfortunate if anybody uses their position in public life to exert undue influence on a democratic process such as planning.
"There appears to be evidence that he has written behind the scenes both about planning applications and also about the appointment of particular architects, which would be an abuse of his position, definitely."
Clarence House declined to comment on the claims.
In the past, Prince Charles has not been shy about airing his opinions on architecture.
He caused controversy recently when he intervened on plans for the Chelsea barracks site in west London, urging developers to reconsider the modern design they had chosen.