Some troubled countries may be shown the exit door to the eurozone, according to Otmar Issing, a former European Central Bank (ECB) chief economist and an architect of the single European currency.

"Everything speaks in favor of saving the euro area. How many countries will be able to be part of it in the long term remains to be seen," Issing writes in his new book, "How We Save the Euro and Strengthen Europe," The Telegraph reports.

Issing didn't mention any countries by name, though fears have been building for some time that Greece will eventually default on its sovereign debt and abandon the euro, possibly pressuring the larger Spain to follow suit.

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Greece has agreed to painful austerity measures such as layoffs and spending cuts in exchange for bailout money.

"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," Issing adds.

Turning to solutions, Issing rejected a call for all eurozone countries to underwrite and issue a single bond to help finance their debt-ridden neighbors.

Germany has balked at the proposal saying fiscal reforms are needed in the debt-ridden countries themselves, adding it cannot ask its taxpayers to shoulder more southern European debt, which would also raise interest rates at home.

"There is no quick fix and anything in the direction of euro bonds or something similar would mean for me the end of the stability-oriented currency union," Issing says.

"The less politicians address the root of the problems, the more they look with their expectations and demands to the ECB, which is not made for this. It is a central bank and not an institution to rescue governments threatened by bankruptcy," Issing notes.

"A central bank always also acts as a lender of last resort for the banking system — but it does not rescue governments."

Greece, meanwhile, has said it will stick with austerity measures in exchange for continued bailout payments arranged by the European Commission, the ECB and the International Monetary Fund.

The country must present to creditors a plan to shave 4 billion euros off spending as part of a broader commitment to slash 11.5 billion euros from its 2013-14 budget.

"We are not there yet, we still have 3.5 billion euros to 4 billion euros to cover," said Finance Minister Yannis Stournaras, according to Reuters.

Editor's Note: Unthinkable Haunts Investors: Evidence for Imminent 90% Stock Market Drop.