Europe’s common currency union was formed more than a decade ago and now includes 17 European Union members, creating a powerful economic bloc aimed at cementing stability on the Continent. It ushered in years of prosperity for its members, especially Germany, as interest rates declined and money flooded into the union — until the Lehman Brothers bankruptcy sent global credit markets into chaos three years ago and the financial crisis took on new life with the near-default of Greece last year. The creation of the euro zone meant countless interlocking contracts and assets among the countries, but no mechanism for a country to leave the union.

But as the crisis leaps to Europe’s wealthier north, banks have been increasing their preparedness for any outcome. For instance, while it would certainly be legally, financially and politically complicated for Greece to quit the euro zone, some banks are nonetheless tallying how euros would be converted to drachmas, how contracts would be executed and whether the event would cause credit markets to seize up worldwide.

The Royal Bank of Scotland is one of many banks testing its capacity to deal with a euro breakup. “We do lots of stress-test analyses of what happens if the euro breaks apart or if certain things happen, countries expelled from the euro,” said Bruce van Saun, RBS’s group finance director. But, he added: “I don’t want to make it more dramatic than it is.”

Certain businesses are taking similar precautions. The giant German tourism operator TUI recently caused a stir in Greece when it sent letters to Greek hoteliers demanding that contracts be renegotiated in drachmas to protect against losses if Greece were to exit the euro.

TUI took the action just days after Mrs. Merkel and President Nicolas Sarkozy of France acknowledged at a meeting earlier this month of G-20 leaders in Cannes, France, that Greece could well leave the monetary union. On Thursday, Greece’s central bank warned that if the country failed to improve its finances quickly, the question would become “whether the country is to remain within the euro area.”

In a survey published Wednesday of nearly 1,000 of its clients, Barclays Capital said nearly half expected at least one country to leave the euro zone; 35 percent expect the breakup to be limited to Greece, and one in 20 expect all countries on Europe’s periphery to exit next year.

Some banks are now looking well beyond just one country. On Friday, Merrill Lynch became the latest to issue a report exploring what would happen if countries were to exit the euro and revert to their old currencies. If Spain, Italy, Portugal and France were to start printing their old money again today, their currencies would most likely weaken against the dollar, reflecting the relative weakness of their economies, Merrill Lynch calculated.