Charles celebrates his 70th birthday with a series of celebrations to commemorate the Prince's service in the royal family.
The Prince of Wales, which is the longest heir to the throne, has been under investigation, as he is set to take the Queen's seat as the government monarch.
Charles has claimed to be a "shadow king" under the queen, and is already preparing his role as a monarch in 2019, criticism from Republic.org has said.
However, they insisted that the prince must face "proper review" by the public, otherwise he is "not appropriate" to become king.
They have also pointed to how the majority of the British public oppose Charles taking the Queen's seat on the throne.
Critics from Republic.org wrote: "Charles is now regularly referred to as a" Shadow King "who quietly takes over from the Queen, but we say every head of state is worth serving the British people to get out of the shadows and meet the right review – media and on the voting box.
"If he is not willing to do that, he is not appropriate to be our head of state. Not only that, the vast majority does not want Charles to be king."
According to a recently asked YouGov poll, only 48 percent of the British public has a positive opinion Charles compared to his mother, the queen, who has a 74 percent rating.
Meanwhile, 20 percent of the polls had a negative view of Charles, while only nine percent of the polls saw the Queen negatively.
Critics have surrendered to Charles as the future king, insisting that his government to the throne must be "challenged".
Republic.org wrote on their website: "Talk of the" Shadow King "tells us that the transition to King Charles has already begun, and we have a limited time to stop it.
"It means challenging Charles and being at the forefront of the debate about what kind of head of state we want to follow the Queen's reign.
"We will win this debate by creating an active movement that forms an extensive public debate on the future of the monarchy."
Charles is ranked as the seventh most popular royal among the British public, according to a new study by YouGov.
At the same time, his son, Prince Harry, peaked the list with 77 percent positive grades, followed by the Queen, Prince William, Kate, Duchess of Cambridge, Prince Philip and Meghan Markle.
The Duchess of Cornwall was ranked much lower on the list, with only 29 percent of the polls who had a positive opinion of her.
Camilla is ranked 10th on the list, before Sophie, the Countess of Wessex and Prince Edward, Earl of Wessex who rank 11 and 12 respectively.