Cameron made the remarks as he warned that the UK's own economic recovery would be slow and painful, not least because of the situation in the other major European economies.
The eurozone crisis is not even halfway through and the single currency could yet break up, David Cameron has suggested in a television interview.
The UK Prime Minister said that the euro economies faced "a very long and painful process" lasting several more years.
Cameron made the remarks as he warned that the UK's own economic recovery would be slow and painful, not least because of the situation in the other major European economies.
His remarks about the euro are likely to raise eyebrows in other European Union capitals, where some leaders have been publicly irritated by British warnings about the future of the euro.
Eurozone leaders have struggled with a debt crisis and lack of confidence in financial markets for at least three years.
Despite a slight easing of it this year, thanks to European Central Bank support, fears are again growing. Spain, Greece and other eurozone members are trying to balance budgets through cuts in public spending and tax increases, but the austerity programmes are undermining economic growth.
In an interview on BBC One's The Andrew Marr Show, Cameron said: "I don't think we're anywhere near halfway through it because what's happening in the eurozone is a massive tension between the single currency that countries are finding very difficult to adapt to, and that's what we're seeing."
The countries using the euro have not yet answered fundamental questions about whether the currency union can continue in its current form, he said.
Cameron said the situation elsewhere in Europe had played a large part in the UK slipping back into recession.
Ed Balls, the shadow chancellor, accused Cameron of "desperate attempts to blame the eurozone" and said the coalition's austerity programme was the real cause for recession.
He said: "No wonder voters are now starting to question economic policies that are hurting, but not working."
Cameron said there could be no departure from the austerity programme because it would push up interest rates.
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