Without reforms, it can be difficult to significantly reduce spending, and S&P said France has already maxed out its ability to raise taxes. That means the government will also struggle to improve its public finances, even more so as the agency expects French unemployment to remain above 10 percent until 2016.
"The overall effect appears to us to leave France with less economic flexibility than other highly rated eurozone members," the agency said.
The market reaction to the downgrade in bond markets was fairly muted, with the country's 10-year bond was up a bare 0.02 percentage point at 2.18 percent still a historically low rate.
That may be because repeated downgrades have taken the sting out. S&P was the first agency to strip France of its top AAA rating last year. The other two major ratings agencies, Moody's and Fitch, have since followed suit.
French Finance Minister Pierre Moscovici said he regretted S&P's decision but emphasized that France's credit rating remained among the highest in the European Union and the world.
"I deplore certain judgments that, I think, are inexact criticisms," Moscovici told France Info radio. "They underestimate the ability of France to transform itself, to right itself."