The Europe of Nations and Freedom (ENL) group of European deputies also includes the Italian anti-immigrant League party, led by Matteo Salvini who hopes to head the next Italian government following Sunday's general election there.
According to a written opinion by the EU parliament's budget control committee, the ENL group has, since 2016, insufficiently justified €38,889 of expenses and violated the rules of tender on another €388,278 euros.
The expenses in question include meals of more than €400 per person and a hundred Christmas presents of more than €100 each.
The budget watchdog also suggests that the far-right grouping in the EU parliament hand back money used to buy some 230 bottles of champagne.
FN deputy secretary general Nicolas Bay told AFP that it was all a "question of interpretation of the rules", adding that there was "no wish to break the rules".
The ENL has "conformed with the tightest interpretation" of the parliamentary rules since 2017, he added.
The parliamentary document remains, at this stage, merely advisory and could be amended up until March 12 when it will be submitted to the European parliament office, which includes the parliament's chief Antonio Tajani.
A decision will be made thereafter as to whether to bill the far-right group for payments deemed to be unjustified.
This latest claim follows reports on Sunday that judges are investigating whether French far-right leader Marine Le Pen and her party illegally took more EU parliament money to pay for France-based staff than previously thought.
If confirmed, the claims could cast a pall over the National Front congress next weekend aimed at recasting the party's image after a series of setbacks since Le Pen lost to Emmanuel Macron in last year's presidential race.
The party is suspected of using money from Brussels earmarked for parliamentary assistants to pay staff for party work in France.
The Journal du Dimanche newspaper said the two French judges looking into the suspected EU funding scandal now think it may have been carried out from July 2009 into 2012.
That would mean nearly seven million euros was potentially misused, up from a previous estimate of some five million euros.