It was vital sportsmen and pop stars denounced illegal substances, he added.
Celebrities should not think their position put them "above the law", said the prime minister.
He was speaking at a "citizen's jury" event in central London which was broadcast to 400 parents, teachers, children and social workers across the country.
Mr Brown said Dame Kelly had agreed to support an initiative to speak out against drugs, and called for more high-profile celebrities to do the same.






