Cameron discussed with the visiting Italian premier his plans to renegotiate the UK's relationship with the 28-member bloc ahead of a referendum on Britain's membership of a reformed EU in 2017.
"We have a lot of time to discuss but in this moment I believe it is absolutely crucial for Italy the presence of UK in Europe, not only for the past of UK but for the future of Europe," Renzi said in English at a joint press conference.
Switching to Italian, the centre-left leader said through a translator: "There is no great Europe without the presence of the United Kingdom, without David Cameron and his compatriots.
"So it is essential for us in this path, this complex path which we have in front of us, the UK presence in Europe is not questionable, it is essential and crucial for us and we are going to work together I am sure."
But when pressed by reporters on the issue of treaty change, Renzi said his priorities were reforming the Italian political system and economy before worrying about wider EU issues.
"At this moment in Italy the priority is a different idea of the future of our country," he said. "The rest we discuss in the future."
Renzi, Italy's youngest ever prime minister at 39, took power in February after engineering an internal party coup against former premier Enrico Letta.