A flea market in Athens. In a survey this summer, an advisory firm found that 80 percent of clients polled expected Greece to leave the euro zone, and a fifth of those expected more to follow.

Bank of America Merrill Lynch has looked into filling trucks with cash and sending them over the Greek border so clients can continue to pay local employees and suppliers in the event money is unavailable. Ford has configured its computer systems so they will be able to immediately handle a new Greek currency.

No one knows just how broad the shock waves from a Greek exit would be, but big American banks and consulting firms have also been doing a brisk business advising their corporate clients on how to prepare for a splintering of the euro zone.

That is a striking contrast to the assurances from European politicians that the crisis is manageable and that the currency union can be held together. On Thursday, the European Central Bank will consider measures that would ease pressure on Europe’s cash-starved countries.

JPMorgan Chase, though, is taking no chances. It has already created new accounts for a handful of American giants that are reserved for a new drachma in Greece or whatever currency might succeedthe euro in other countries.

Stock markets around the world have rallied this summer on hopes that European leaders will solve the Continent’s debt problems, but the quickening tempo of preparations by big business for a potential Greek exit this summer suggests that investors may be unduly optimistic. Many executives are deeply skeptical that Greece will accede to the austere fiscal policies being demanded by Europe in return for financial assistance.

Greece’s abandonment of the euro would most likely create turmoil in global markets, which have experienced periodic sell-offs whenever Europe’s debt problems have flared up over the last two and a half years. It would also increase the pressure on Italy and Spain, much larger economic powers that are struggling with debt problems of their own.

“It’s safe to say most companies are preparing,” said Paul Dennis, a program manager with Corporate Executive Board, a private advisory firm.

In a survey this summer, the firm found that 80 percent of clients polled expected Greece to leave the euro zone, and a fifth of those expected more countries to follow.

“Fifteen months ago when we started looking at this, we said it was unthinkable,” said Heiner Leisten, a partner with the Boston Consulting Group in Cologne, Germany, who heads up its global insurance practice. “It’s not impossible or unthinkable now.”

Mr. Leisten’s firm, as well as PricewaterhouseCoopers, has already considered the timing of a Greek withdrawal — for example, the news might hit on a Friday night, when global markets are closed.

A bank holiday could quickly follow, with the stock market and most local financial institutions shutting down, while new capital controls make it hard to move money in and out of the country.

“We’ve had conversations with several dozen companies and we’re doing work for a number of these,” said Peter Frank, who advises corporate treasurers as a principal at Pricewaterhouse. “Almost all of that has come in over the transom in the last 90 days.”

He added: “Companies are asking some very granular questions, like ‘If a news release comes out on a Friday night announcing that Greece has pulled out of the euro, what do we do?’ In some cases, companies have contingency plans in place, such as having someone take a train to Athens with 50,000 euros to pay employees.”

The recent wave of preparations by American companies for a Greek exit from the euro signals a stark switch from their stance in the past, said Carole Berndt, head of global transaction services in Europe, the Middle East and Africa for Bank of America Merrill Lynch.

“When we started giving advice, they came for the free sandwiches and chocolate cookies,” she said jokingly. “Now that has changed, and contingency planning is focused on three primary scenarios — a single-country exit, a multicountry exit and a breakup of the euro zone in its entirety.”

Banks and consulting firms are reluctant to name clients, and many big companies also declined to discuss their contingency plans, fearing it could anger customers in Europe if it became known they were contemplating the euro’s demise.

Central banks, as well as Germany’s finance ministry, have also been considering the implications of a Greek exit but have been even more secretive about specific plans.

But some corporations are beginning to acknowledge they are ready if Greece or even additional countries leave the euro zone, making sure systems can handle a quick transition to a new currency.

In Europe, the holding company for Iberia Airlines and British Airways has acknowledged it is preparing plans in the event of a euro exit by Spain.

“We’ve looked at many scenarios, including where one or more countries decides to redenominate,” said Roger Griffith, who oversees global settlement and customer risk for MasterCard. “We have defined operating steps and communications steps to take.” He added: “Practically, we could make a change in a day or two and be prepared in terms of our systems.”

In a statement, Visa said that it too would also be able to make “a swift transition to a new currency with the minimum possible disruption to consumers and retailers.”

Juniper Networks, a provider of networking technology based in California, created a “Euro Zone Crisis Assessment and Contingency Plan,” which company officials liken to the kind of business continuity plans they maintain in the event of an earthquake.

“It’s about having an awareness versus having to scramble,” said Catherine Portman, vice president for treasury at Juniper. The company has already begun moving funds in euro zone banks to accounts elsewhere more frequently, while making sure it has adequate money and liquidity in place so employees and suppliers are paid without disruption.

FMC, a chemical giant based in Philadelphia, is asking some Greek customers to pay in advance, rather than risk selling to them now and not getting paid later. It has also begun to avoid keeping any excess cash in Greek, Spanish or Italian bank accounts, while carefully monitoring the creditworthiness of customers in those countries.

“It’s been a very hot topic,” said Thomas C. Deas Jr., an FMC executive who serves as chairman of the National Association of Corporate Treasurers. Members of his group discussed the issue on a conference call last Tuesday, he added.

American companies have actually been more aggressive about seeking out advice than their European counterparts, according to John Gibbons, head of treasury services in Europe for JPMorgan Chase.

Mr. Gibbons said a handful of the largest American companies had requested the special accounts configured for a currency that did not yet exist.

“We’re planning against the extreme,” he said. “You don’t lose anything by doing it.”