She has maintained a public silence on politics and public debate in nearly seven decades on the throne.
Queen Elizabeth's weekly briefings from prime ministers — who run the country — remain private and her public roles as a charity patron are ceremonial. While the monarch is Britain's official head of state, parliament has been in charge since the late 17th century.
By contrast, Charles is an outspoken campaigner on issues ranging from organic and traditional farming and wildlife conservation to herbal medicine and urban planning.
The BBC asked the prince whether his public campaigning would continue in the event of his mother's death.
"No, it won't. I'm not that stupid," said Charles, who is officially known as the Prince of Wales. "I do realize that it is a separate exercise being sovereign. So of course I understand entirely how that should operate."
Asked about accusations that he had been "meddling" in public issues, he responded: "If it's meddling to worry about the inner cities as I did 40 years ago, then if that's meddling I'm proud of it."
Ahead of Prince Charles' 70th birthday next week, here's a look at his life in pictures with a classic image of the heir to the throne for each year pic.twitter.com/BDxu52Q0TO