LONDON — A leading credit ratings agency warned on Monday that Greece would be considered to be in default if banks rolled over their holdings in the country’s debt as proposed recently in a French plan. Standard & Poor’s said in a statement that two proposals by an association of French banks “would likely amount to a default” under its criteria because both options offer “less value than the promise of the original securities.”

S&P’s position could wreak havoc on Europe’s attempts to deal with the Greek debt crisis, especially if rivals Moody’s and Fitch come to the same conclusion. A so-called “selective default” could trigger insurance claims on Greek bonds and cause another bout of turmoil in the financial markets.