Marine Le Pen has been charged over claims her party illegally claimed millions of Euros from the European Parliament.
The controversy shadowed Le Pen's campaign for the French presidency. She was then charged in a process known as "mise en examen" triggered when there is serious and consistent evidence showing likely involvement in a case. She will appeal the decision.
Le Pen has already had her parliamentary immunity lifted over a separate case from 2015 when she shared graphic pictures of atrocities committed by the Islamic State group on her Twitter account.
Le Pen's chief of staff Catherine Griset was put under formal investigation on February 22 after a day of questioning over the alleged misuse of European Union funds to pay parliamentary assistants.
Marine Le Pen formally investigated in EU funding inquiry
The formal investigation of Le Pen comes less than two weeks after she won a seat in the French National Assembly. At the time, Le Pen refused to answer questions about the alleged wrongdoing. The FN leader has previously denied wrongdoing.
Investigators suspect some National Front lawmakers used legislative aides for the party's political activities while they were on the European Parliament payroll.
The anti-EU nationalist beat the candidates of the traditional right and left to secure a spot in May's presidential run-off against the pro-EU centrist Emmanuel Macron.
Le Pen denies the charges against her, but is now under active investigation concerning the appropriation of at least €5m ($5.4m). Instead, she lost 66% to 34%.
Sony is to start making vinyl records again
In fact, they're popular enough in Japan that Sony is preparing to restart mass production after almost 30 years. According to stats physical music sales represented 73% of total record industry revenues in Japan a year ago .