The ruling means he'll lose out on a reimbursement of more than $12.8 million, Radio France Internationale reported Friday.
A financial watchdog in France said Sarkozy overspent during last year's election campaign by 2.1 percent. Because of that, he'll lose out on a 47.5 percent reimbursement and must return $193,000 that the state advanced him during the campaign.
Sarkozy resigned from the country's top constitutional body in order to "regain his freedom of speech" so he could react to the decision, Radio France Internationale reported.
Sarkozy said on his Facebook page Friday the decision put his center-right UMP party "in peril" due to its continuing decline in resources.
UMP party leader Jean-Francois Cope called for a "massive national contribution" to help salvage the party's finances, Radio France Internationale said.