"Yes we are a small island, in fact a group of small islands, but I would challenge anyone to come up with a country with a prouder history, with a bigger heart or with greater resilience," Cameron told reporters at the two-day G20 summit in St. Petersburg.
Cameron made the impassioned remarks at a morning news conference and then repeated much of what he said in a subsequent gathering of reporters.
It was in response to a Russian official who reportedly told a group of journalists earlier this week that Britain was "just a small island that no one pays any attention to."
"This is a country that cleared the European continent of fascism, that took slavery off the high seas. We're a country that's invented many of the things that are most worthwhile — everything from the Industrial Revolution to the television to the World Wide Web," Cameron said.
"Our music delights and amuses million. The Beatles, Elgar and — slightly less congruously — One Direction have conquered the world."
He added that Britain also invented most of the sports the world most likes playing.
"If I go on too long about our literature, our art, our philosophy, our contribution, including of course the world's language — if I start talking about this blessed plot, this sceptered isle, this England, I might have to put it to music, so I think I'll leave it there," he said, quoting from Shakespeare's Richard II.
Cameron did say that Russia has denied making the comments.