European efforts to encourage private investors to contribute to a new international bailout for Greece hit new problems Monday when Standard & Poor's Ratings Services said the leading proposal under consideration would likely bring the country into default.

The comments from S&P are the first reaction from a rating company to a preliminary plan that emerged last week from French banks—the biggest foreign holders of Greek debt. The plan aims to encourage holders of Greek government bonds maturing between now and 2014 to roll...