Channel 4 News pulled out of an interview with Prince Charles because of control Clarence House tried to exert over it that campaigners said were "North Korea-style".
The programme pulled out of an interview, due to be conducted by presenter Jon Snow last Sunday in Paris, ahead of the climate change talks, a subject on which the Prince of Wales has been vocal.
Clarence House requires media who want to interview the Prince of Wales to sign a 15-page contract that gives the household the right to view in advance the final cut of any broadcast material, The Independent reported today.
It was accused of "co-opting our national broadcasters" by anti-monarchists and using them as an "extension of his PR operation".
Channel 4 News declined to comment.
When asked about contracts entered into with media, a Clarence House spokeswoman did not comment on the details in The Independent, which also reported the contracts allowed it to approve all questions in advance.
In a statement, she said: "The issuing of broadcast contracts is standard practice across the Royal Households.
"All broadcasters who enter into them are keen to ensure that they do not breach any of the relevant broadcast rules and go to great lengths to protect their independence in this regard.
"The contracts are put in place to ensure factual accuracy and protect the broadcaster as well as the interview subject."
Prince Charles was interviewed by Sky News last week. During that interview, he said climate change was one of the causes of the Syrian civil war.
"There's very good evidence indeed that one of the major reasons for this horror in Syria, funnily enough, was a drought that lasted for about five or six years, which meant that huge numbers of people in the end had to leave the land," he said.
"We're seeing a classic case of not dealing with the problem, because, I mean, it sounds awful to say, but some of us were saying 20 years ago that if we didn't tackle these issues, you would see ever greater conflict over scarce resources and ever greater difficulties over drought, and the accumulating effect of climate change."
Sky News declined to comment on whether it had signed a pre-broadcast contract.
Graham Smith, chief executive of anti-monarchy campaigners Republic, said: "Prince Charles has effectively co-opted our national broadcasters, using them as extensions of his PR operation.
"Serious questions need to be asked of our broadcasters about why they have colluded with the royals in this way."
He said: "This is North Korea-style control and censorship. Prince Charles wants to be able to say whatever he wants, and he wants to control how his comments are reported and who responds to them.
"These contracts raise serious questions about the editorial independence of our national broadcasters. Any broadcaster signing these contracts should hang their heads in shame."
Prince Charles is more vocal on current affairs than his mother. A series of letters he wrote to ministers were subject to a 10-year battle to release them under Freedom of Information laws.
The letters, on issues from defence supply to the fate of the Patagonian Toothfish, were published in May.
Smith added: "Prince Charles should not be wading into public debates. If he wants to then he should be subjected to the same level of scrutiny and challenge as anyone else.
"If Prince Charles is not prepared to be challenged in the way that an MP might be expected to be challenged then he should keep his opinions to himself."