The 20-year-old recording artist is expected to be seen in the commercials starting next spring, the Daily Mail reported.
A Cadbury spokesman said Stone was selected because "she reflects the Flake girl's attitude to life."
Some of the company's previous TV spots have sparked controversy for being too sexy. One ad was yanked off the air in the 1970s after people complained and Cadbury stopped running the commercials in 2004.
The company decided to start airing them again this year as a means to boost the candy bar's melting sales, which dropped 15 percent in two years, the newspaper said.
Notable deaths of 2014 [PHOTOS]
Mindy Kaling mistaken for Malala Yousafzai