Last week, No. 1 U.S. automaker GM asked U.S. District Judge Paul Borman to reopen the case, claiming it had new information on foreign accounts used in an alleged bribery scheme involving FCA and leaders of the United Auto Workers (UAW) union.

When it first filed its lawsuit last year, GM alleged that FCA bribed UAW officials over many years to corrupt the collective bargaining process and gain advantages, costing GM billions of dollars. GM was seeking "substantial damages" that one analyst said could have totaled at least $6 billion (4.6 billion pounds.

But last month, Borman threw out the racketeering lawsuit, saying GM's alleged injuries were not caused by FCA's alleged violations.

In its latest filing, FCA said that as it operates facilities in Italy and more than 40 other countries, the existence of foreign bank accounts is "unremarkable, and certainly not illegal."

After FCA's filing, GM said it intends to uncover "the full extent of harm the FCA bribery scheme caused GM."

"FCA's corruption of the collective bargaining process remains undeniable," the automaker said in a statement.

FCA is due to merge with France's PSA by the first quarter of 2021.

"It is... clear to me that this series of attacks is directly related to our success in competing and winning where it matters, in the market," FCA Chief Executive Mike Manley wrote on Monday in a letter to employees seen by Reuters. "The consistent strengths we've demonstrated over the last decade will be deployed to even greater effect as we complete our merger with Groupe PSA."

By Nick Carey