The eurozone is doomed unless Germany agrees to underwrite the debts of struggling members, Tony Blair warned today.
The former Prime Minister said the problems were now so serious that a "grand plan" was the only way to prevent a break-up.
However, he also suggested that Britain could still join the single currency area if it stabilised.
The intervention came as EU leaders prepare for a crunch summit this week that could determine the fate of the eurozone.
The difficulties intensified this morning when it emerged that newly-elected Greek Prime Minister Antonis Samaras will miss the event after undergoing eye surgery.
Speaking on the BBC's Andrew Marr show, Mr Blair said: "The only thing that will save the single currency now is in a sense a sort of grand plan in which Germany is prepared to commit its economy fully to the single currency.
"That means treating the debts of one as the debts of all, which is very hard for Germany to do.
"It means the other countries in the eurozone need to reform, need precise credible programmes of change to reform so that Europe can regain its competitiveness.
"Otherwise it is quite unfair to ask Germany to pay."
But Mr Blair - a prominent Europhile - refused to accept that the single currency's troubles meant the UK would never join.
"If they sort it all out and Europe moves forward again then Britain is going to have a very interesting choice to make," he said.
Mr Blair said he had never disagreed with Gordon Brown on the economics of euro membership - despite claims that the then-chancellor was responsible for blocking the UK's entry.
"I always took the view that economically you had to make an unambiguous case for Britain joining," he said. "Politically however, I was always in favour of keeping us very positive towards the project of European integration, and able to join at any time we wanted to do so."
The former premier said the "broad sweep of history" showed that European integration would continue regardless of the fate of the eurozone. Britain needed to protect its alliance with the EU in order to exercise influence, he added.
He also appeared to hint that he would be interested in the role of EU president in the future.
"I have always said I am a public service person first. I would have been happy to carry on as Prime Minister, I would have been happy taking the European job as president of the EU," Mr Blair added.