David Cameron pledged he will not ask British taxpayers to underwrite the debts of ailing banks in Greece and Spain, as he held talks with German chancellor Angela Merkel about the eurozone crisis.
The Prime Minister said he had "no doubt" that the 17 nations of the eurozone would move towards closer fiscal union within the next weeks and months.
But he made clear that Britain would not be involved in any such arrangement, which could involve the creation of eurobonds to spread the risk of borrowing across the whole single currency area.
Mr Cameron said: "I can understand why eurozone countries may want to look at elements of banking union.
"Because we are not in the single currency, we won't take part in the profound elements of that banking union.
"I wouldn't ask British taxpayers to stand behind the Greek or Spanish deposits.
"It is not our currency, so that would be inappropriate to do.
"I understand why single currency countries have to look at deeper integration.
"I will make sure that Britain's interests, particularly in the single market and the openness and fairness of the single market are protected.
"That is key for Britain.
"We want the eurozone to succeed. We want the euro to solve the problems it faces, so that all European economies including ours can get back to healthy growth."
Mr Cameron said the talks were "good and positive" but he had no announcement to make of any breakthrough in the battle to save the euro.
In the run-up to the talks in Berlin, the PM said that "speed is of the essence" in restoring stability and market confidence in the euro, and agreed with US President Barack Obama that an "immediate plan" was needed.
But with re-run Greek elections due on June 17, the meeting was never expected to do anything more than pave the way for summits of the G20 in Mexico and European Council in Brussels.
"It is not easy when you have got in the eurozone 17 countries, 17 governments and one currency, so I understand the difficulties," he said.
"But obviously, I am pressing the case for action to solve the financial crisis, to recapitalise the banks, build the big firewall, get growth going in Europe through structural reform and make sure there are clear and credible plans to deal with deficits."