A French watchdog that checks he probity of public figures has asked the prosecutor's office to look into the far right's top two figures — Jean-Marie Le Pen and Marine Le Pen — after concluding that they may have underestimated their real estate assets.

The High Authority for the Transparency of Public Life said on Monday that its examination of the declarations of the pair led to a "serious doubt about the exhaustiveness, the exactitude and the sincerity" of the files.

The announcement comes a week after the National Front failed to make the big breakthrough it had hoped in regional elections.

Jean-Marie Le Pen founded the party and his daughter leads it.

The two Le Pens said separately they're appealing the decision before the Council of State.