Some members of the eurozone may have to leave the bloc as the debt crisis continues, according to one of the architects of the euro.
"Everything speaks in favour of saving the euro area," said Otmar Issing, a former European Central Bank chief economist.
"How many countries will be able to be part of it in the long term remains to be seen," he added.
His message comes as the ECB warned of a lack of lending within the region.
The central bank highlighted the fall-off in money being lent across borders between the 17 countries that share the euro, saying the eurozone is becoming increasingly fragmented.
The ECB said cross-border loans in the overnight money market fell to 40 per cent of total loans, from 60 per cent in a similar period last year.
A recent report by Morgan Stanley focused on the "Balkanisation" of the eurozone banking system.
"Its banking and money team have highlighted how the weaker eurozone economies — Spain, Italy, Portugal, Cyprus, Greece and Ireland — have been progressively starved of credit as banks in the bigger, stronger economies of Germany and France have stopped lending to them," the BBC's business editor Robert Peston said.
Mr Issing was a member of the German Bundesbank until 1998 and then worked at the ECB, during the introduction of the euro in 1999 until 2006.
Promoting his new book on saving the euro, Mr Issing said: "We are still a long way off saying "that's it, now we are sure to make progress".
"Substantial reforms in almost all countries are still pending."
He added it was not true that Germany would be better off returning to the deutschmark, saying the euro had been more stable than the mark.
"One should focus on bringing the euro back to what it was meant to be: a stable currency."