Anne-Christine Poujoulat, AFP | Jean-Luc Melenchon addresses the press and his supporters in Marseille on 18 June 2017 - Reuters
France - 19 July 2017: The preliminary investigation - already targeting members of France's centrist MoDem party, conservative party The Republicans and the Socialist Party - was opened after a member of Marine Le Pen's far-right National Front asked the Paris prosecutor to look into the issue.
Le Pen is herself under formal investigation for breach of trust in a separate case on the same subject.
According to Le Parisien daily newspaper, three people who were Melenchon's parliamentary aides while he was a member of the European Parliament from 2009 to 2017, are to be investigated.
Being a target of a preliminary investigation or a formal investigation in France does not necessarily lead to a trial.
Melenchon, now a member of the French parliament, leader of the France Unbowed party and a vocal opponent of the government of centrist Emmanuel Macron, denied any misconduct in a weekly briefing on Tuesday afternoon.
Political probity became a major issue during this year's presidential campaign in which Le Pen and Melenchon were both high-scoring candidates, eventually losing to centrist Emmanuel Macron.