Asked if he thought the euro zone would remain intact, Cameron told Sky News: "I think the most likely outcome is yes, it will hold together, but it has to take some pretty decisive steps."
These included short-term "sticking plaster" steps such as a proper firewall to prevent contagion around Europe and a much more decisive settlement for Greece, he said.
"The longer term is you've got to address the fact there's a
lack of competitiveness between Germany on the one hand and many of the southern European countries on the other," he said.
(Reporting by Adrian Croft)