PARIS — Greece risks being judged in default on its debt obligations if banks are forced to bear part of the pain, Standard & Poor’s said Monday, suggesting that current proposals for rescuing the euro zone’s weakest member may have to be reconsidered.

In particular, a plan proposed by the French government and banks “could require private sector debt restructuring in a form that we would view as an effective default,” S.&P. said in a statement.

The effects of a Greek default would be felt around the world. The country's debt of €330 billion might not be large enough in itself to set off a renewed financial crisis, but once the precedent of a euro-zone default had been set, investors would likely abandon the debts of other struggling members, including Portugal and Spain.

More worryingly, Western banks, including the giants of Wall Street, have built a tower of credit default swaps — essentially insurance — on the debts of those countries, and the cost of paying up in a default would be huge. While the French and German banks have the biggest direct exposure to Greek's debt, it is American banks and insurance companies that would have the largest obligations to cover payments to those holding the swaps.