Le Pen ran her presidential campaign under an aggressively xenophobic, White Nationalist Platform which included aggressive anti-Immigrant policies. Last February, Le Pen's chief-of-staff Catherine Griset was placed under formal investigation by a financial court, while her longtime bodyguard Thierry Legier was questioned.
Le Pen is no longer a member of the European parliament but was elected earlier this month to the French parliament for the first time after winning a seat in the northern, former coal mining region of Pas-de-Calais.
Investigators suspect that the party took money reserved for the salaries of European parliamentary assistants and used it to fund staff working for the National Front.
Le Pen's office said that the sum was going toward paying around 40 parliamentary assistants from 2012 til now.
The 48 year old head of the National Front is being investigated for breach of trust and has denied any wrongdoing.
She was decisively beaten by pro-EU centrist Emmanuel Macron.
Le Pen is also charged with complicity in breach of trust in her role as president of the National Front from 2014 to 2016.
On Monday, Le Pen is to appeal the investigating magistrates' decision which "violates the principle of the separation of powers", according to the defense lawyer. Instead, she lost 66% to 34%.