French presidential frontrunner Emmanuel Macron filed a complaint Thursday after his far-right rival Marine Le Pen implied he had an "offshore account in the Bahamas", an aide said.
"We will not hesitate to prosecute for defamation anyone who repeats this false information," a member of Macron's camp said.
During Wednesday's ferocious televised debate Le Pen said: "I hope that we will not find out that you have an offshore account in the Bahamas." Macron refuted the suggestion as "defamation".
Judicial sources said prosecutors in Paris had opened a probe following the complaint from Macron, which comes three days before Sunday's presidential run-off against Le Pen.
According to a source close to the case, the complaint targets "information that circulated Wednesday night on the internet" alleging tax evasion in the Bahamas.
Speaking on French radio Thursday, the centrist, pro-EU candidate characterised the insinuations as "fake news and lies" from "sites, some of which were linked to Russian interests".