As populist politicians who defend the “real people of France” against a hypocritical élite, the Le Pens have always been reluctant to discuss their wealth. But the father and daughter now face a formal investigation for allegedly underestimating their personal fortunes by 60 per cent in an official declaration last year.

If the claim is found to be true, Marine Le Pen, leader of the far-right Front National, could be banned from politics for a decade – transforming the French political landscape and changing calculations about the next presidential election in 2017. She could even, in theory, be sent to jail.

France's far-right political party Front National (FN) founder Jean-Marie Le Pen on stage as Marine Le Pen looks on (AFP)

Neither outcome is likely. All the same, the complaint, made by the “high authority for openness in public life”, is potentially embarrassing for Jean-Marie and Marine Le Pen. Both have denied the allegations.

The far-right patriarch, Jean-Marie, 87, accused the “establishment” of “harassing” him. Marine, 47, has launched a counter-suit accusing the “high authority” of “abuse of power”. The watchdog says its investigations have raised “serious doubts” about the “precision and sincerity” of the financial declarations made in 2014. According to press leaks, Mr Le Pen is suspected of under-declaring his fortune by €1m and Marine Le Pen by several hundred thousand euros.

The disputed figures mostly concern the value of family properties, including a €6m château in Saint-Cloud, west of Paris, which was inherited by Le Pen senior from a cement millionaire and political supporter in 1977. Marine Le Pen, who lives in part of the château, owns part of a company to which her father transferred the building’s ownership in the early 1990s.

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The watchdog is also looking into whether the Le Pens under-declared the value of their shares in other property companies – including one that owns the former Front National headquarters in Saint-Cloud.

The investigation is the latest in a series of financial embarrassments for the Le Pens. The Front National is the subject of a criminal investigation over allegations of “fraud and embezzlement” reportedly relating to over-charging its own candidates for election materials in 2012.

Jean-Marie Le Pen is the subject of a tax-evasion investigation for allegedly keeping an undeclared bank account, including €1.7m in gold ingots and coins, in Geneva. Both the Front National and Mr Le Pen deny these accusations.

The requirement that French politicians declare their wealth is new. It was imposed in 2013 after Jérôme Cahuzac, the Socialist budget minister and the government’s main campaigner against tax fraud, was found to have an off-shore bank account. Any politician who fails to declare their wealth fully faces a €45,000 fine, up to three years in prison and a ten-year period of “ineligibility” to stand for office.