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French Foreign Minister Alain Juppe said it appeared to him that S&P had made its decision before Mrs. Merkel and Mr. Sarkozy released details of the new plan, so the ratings agency hadn’t been able to factor that into its considerations.

The leaders’ proposal is “exactly the response to one of the major questions from the ratings agency, which talks about insufficient European economic governance,” Mr. Juppe said on RTL radio.

Mr. Sarkozy and Mrs. Merkel are proposing several broad changes for the EU treaty, including the introduction of a penalty for any government that allows its deficit to exceed 3 percent of gross domestic product. The penalty would be automatic — unless a majority of nations opposed it, a loophole that drew sharp criticism from analysts.

Some analysts also feel the proposal, which demands strict austerity measures, misses the mark and only will worsen much-needed growth in already feeble economies.

Investors are hoping that the summit of European leaders on Thursday and Friday will produce concrete measures to prevent a messy breakup of the euro. Markets have been jittery because of fears that the euro might disintegrate, causing a sharp recession in Europe that would spread through the world economy.

EU spokesman Amadeu Altafaj Tardio said that the bloc needed to make “important decisions this week” but not because of any worries about the S&P ratings.

“The job was already partially done in October” at the last summit, he said. “We now have to complete the job. It is not because we want to please the rating agencies or market forces; it is important because it is the best (way) to ensure the prosperity of our citizens.”

The S&P warning left out only two of the 17 countries that use the euro: Cyprus, whose bonds have near-junk status, and Greece, whose low ratings already suggest it is likely to default soon anyway.

Kirsten Grieshaber in Berlin, David Stringer in London, Raf Casert in Brussels and Sarah DiLorenzo in Paris contributed to this article.