Thirty-six per cent of the public say the Prince of Wales has been beneficial to the monarchy, compared to 60 per cent in 2013, according to the survey.
A YouGov poll commissioned by the Press Association found that more than a quarter (27 per cent) think that Charles, who is longest-serving heir to the throne, has had a negative impact on the royals.
Four years ago, that stood at just 15 per cent.
The research also found that only 14 per cent of the British public want the Duchess of Cornwall to become Queen when Charles is King.
Almost a third of Brits (30 per cent) believe she should not have any title at all, while 39 per cent think she should take the title of Princess Consort instead.
Royal commentator Penny Junor said she was surprised by the results.
“We’ve moved on 20 years. Charles’s reputation, I thought, was completely rehabilitated.
“He’s become a much happier, more relaxed and more confident Prince of Wales and I thought much more popular as a result,” she said.
The author of the Camilla biography The Duchess: The Untold Story described Charles as “the most extraordinary Prince of Wales”, adding: “I think he has been visionary over the years, the issues he’s tackled have all become mainstream thinking.”
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But she said: “He gets a hard time because his marriage failed. He’s never been forgiven for that — because Diana blamed him and he never ever said a word.
“He’s never explained what went on in his marriage. His view has always been that history will judge him. He knows that he did all he could to try and make his marriage work and he failed.”
She added: “I do think it’s a very difficult time for him. The question of Diana has always been a source of difficulty between him and his sons.”
The results echo similar polls that suggest the Royal remains unpopular among Britons.
A Sunday Express poll, conducted by Norstat, found 67 per cent of those questioned did not support the Duchess taking the title of Queen when her husband becomes King.
An ICM poll for the Sun revealed 51 per cent wanted the Duke of Cambridge to be the next monarch, while only 22 per cent surveyed said Charles.
Supporters of the heir to the throne point to the sum, said to be more than a billion pounds, he has raised for charity over the past decade, including through work with his Prince’s Trust.
However, Charles’s willingness to meddle in government affairs has contributed to his unpopularity.
The “black spider” memos, letters and memos written by the Prince of Wales, to British government ministers and politicians, released in 2015, were particularly harmful to his reputation.
In the YouGov poll, William came out top for being seen as having a positive contribution to the royal family.
Ranked in order, 78 per cent thought William had a positive impact, Harry 77 per cent, Kate 73 per cent, Charles 36 per cent and Camilla 18 per cent.
When Camilla married Charles 12 years ago, aides said she intended to be known as Princess Consort instead when he is King.
But according to legal experts, unless there is change in the law, Camilla will technically become Queen no matter what title she actually uses.
Clarence House and Kensington Palace declined to comment.