According to a BBC report, a senior Russian spokesman at the G20 summit in St Petersburg had belittled Britain's importance in the world after the decision not to intervene in Syria.
The spokesman dismissed Britain as "a small island that nobody listens to", the report said.
After the BBC first reported this yesterday, President Putin's press secretary and close adviser Dmitri Peskov insisted that "it is definitely not something I have said".
Cameron's office though seemed unconvinced. A spokesman said: "I'm sure the Russians will want to clarify these reported remarks."
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They went on to claim that the G20 agenda reflected "a very British agenda" and "highlighted how a small island with great people can achieve a big footprint in the world."
Cameron joined the defence of his country at a briefing for journalists in St Petersburg, offering a lengthy paean of praise to Britain's historical achievements and contemporary contributions.
"What I've been told is that the Russians absolutely deny making this remark. Certainly no one's made it to me," he was quoted as saying by The Telegraph.
"Britain is an island that has helped to clear the European continent of fascism and was resolute in doing that throughout the Second World War," he said.
"Britain is an island that helped to abolish slavery, that has invented most of the things worth while inventing, including every sport currently played around the world, that still today is responsible for art, literature and music that delights the entire world," the Prime Minister added.