Fiat Chrysler and PSA of France said Wednesday that they had agreed to the terms of a merger that would create the world’s fourth largest automaker.

The companies said they had signed a binding agreement formalizing the merger, announced in October.

The accord brings Fiat Chrysler and PSA, the maker of Peugeot and Citroën cars, much closer to creating a carmaker bigger than General Motors. But there will remain the task of integrating the companies. The companies said Wednesday that it would take another year to 15 months to close the deal.

PSA has shown no sign of second thoughts about the merger even after Fiat Chrysler became the target last month of a racketeering lawsuit by G.M. The complaint asserts that Fiat Chrysler bribed United Auto Workers officials in contract negotiations to get an advantage over G.M. Fiat has called the suit “meritless.”

Carlos Tavares, the head of PSA, said Wednesday that he fully supported Fiat Chrysler in its denial of the accusations. “We have obviously done our due diligence,” he told reporters during a conference call.