The heiress to the fortune of French couturier Nina Ricci on Thursday launched an appeal to avoid jail time for tax fraud, in one of the toughest sentences of its kind in French fiscal history.

Arlette Ricci, 75, was handed a one-year prison term and a million-euro fine in April 2015 for allegedly hiding 18.7 million euros ($19.8 million) from the taxman for more than two decades.

Proceedings at a Paris appeal court on Thursday focused on minor questions of law filed by Ricci's lawyers, and the defendant herself was absent.

Ricci stayed "at home in Switzerland for health reasons," her attorney told AFP, saying it was "unknown at this point" whether she would be able to attend in the future.

The Nina Ricci fashion house, founded in 1932 in Paris, is known for its luxury perfumes -- especially the floral classic L'Air du Temps -- and its classic, slender lines.

The case against Arlette Ricci began after revelations that the Swiss private banking arm of British giant HSBC had helped clients hide billions from the taxman, in what became known as the SwissLeaks scandal.

The authorities are demanding Ricci pay back some 10.5 million euros in unpaid taxes, penalties and fines.

They also confiscated properties in Paris and Corsica, estimated to be worth around four million euros.

Ricci was given a two-year suspended sentence on top of her jail term, but she would only have to serve this if she re-offends.

The sentencing was considered exceptionally tough in France, where tax fraud typically is punished by fines and recovery of claimed sums, but hardly ever by jail. 

But the court said the heiress's actions had posed "an exceptional threat to public order" and that she had disguised her true wealth "for more than 20 years with particular determination."

The appeal is expected to run until December 14.