MARSEILLE, France (Reuters) – France’s highest court threw out on Wednesday a 2015 ruling that cleared a German certifier of negligence after it approved faulty breast implants produced with counterfeit silicone.

The Court of Cassation rejected the ruling by a lower appeals court that had overturned a negligence conviction against TUV Rheinland for its role in approving the implants made by Poly Implant Prothèse (PIP) until the French firm shut in 2010.

PIP sold implants globally over almost two decades until investigators discovered it was passing off low-grade industrial silicone as a much pricier medical product.

The supreme court referred the case to a different appeals tribunal in Paris for a fresh hearing, ruling that TUV Rheinland’s obligations required it to check the implants and documents on the manufacturer’s raw materials, as well as staging surprise visits to PIP.

TUV Rheinland defended the actions of its unit involved in the case. “The fraud committed by PIP was not detectable by TUV Rheinland LGA Products GmbH, and could not be discovered,” TUV Rheinland said in a statement.

The counterfeit substance was used in implants given to 300,000 women. About a quarter of those subsequently removed were found to have ruptured, regulators said, raising concerns over the long-term health effects of exposure to their contents.

PIP founder Jean-Claude Mas was jailed for four years and fined 75,000 euros (now $86,000) in 2013 after a police investigation revealed a sophisticated fraud.

PIP employees would remove evidence of the cheaper silicone gel before annual inspections by TUV Rheinland, it found.

An association representing 15,000 former PIP implant patients said that the supreme court’s ruling set the stage for the “world’s largest collective trial”.

“The Court of Cassation renders a decision in accordance with the law, fair and consistent with the objective elements of the case which are devastating for TUV,” the association’s lawyer Olivier Aumaitre said in a statement.

The association said that a ruling against TUV could cost it as much as 6 billion euros.

($1 = 0.8677 euros)

(Reporting by Jean-Francois Rosnoblet; writing by Leigh Thomas; Editing by Richard Lough and David Stamp)