Investigative news website Mediapart revealed Monday that the elderly founder and former chief of France's far-right National Front party had hidden the money in Swiss bank accounts.
The claims came as Le Pen is engaged with a public feud with his daughter Marine, who now leads the party, which has been facing a string of investigations by both French and European authorities into campaign funding.
Le Pen "kept a hidden account at HSBC, as well as the (private Swiss) CBH bank through a trust placed under the legal responsibility of his butler, Gérald Gérin," the investigative website said.
"Of the €2.2 million placed in the trust account, €1.7 million were kept as gold bars and coins," it added.
Tracfin, an intelligence service that investigates money laundering among other crimes notified prosecutors in the Paris suburb of Nanterre with their suspicions on Monday.
"This notification is going to be examined by prosecutor Catherine Denis and the financial division to decide whether to pursue judicial action," the prosecutor's office said.
Asked about the allegations, Le Pen on Tuesday blasted a "general offensive against" the far-right party.
"I do not need to explain myself over what anyone says, particularly parapolice bodies tasked with sowing disruption in the political class," he told France Inter radio.
According to a source close to the case, who wished to remain anonymous, Le Pen's 2014 declaration of assets is being probed and investigators are focusing on the trust, though there is no proof yet that the 86-year-old is the beneficiary.
The longtime FN leader and current European parliament member has also been under investigation since 2013 over a €1.1-million-euro discrepancy between his 2004 and 2009 asset declarations.
A separate probe also under way focuses on FN campaign financing since Marine Le Pen's takeover in 2011 — with five people and the events company Riwal already charged — while the party has also been referred to the EU's