The German automotive supplier Bosch worked “hand-in-glove” with Volkswagen in the dieselgate emissions scandal, according to court papers filed in the USA.

Lawyers for owners of affected vehicles in the States have filed papers with the US District Court in San Francisco, which allege that Bosch was a “knowing and active participant” in the scheme to dodge emissions controls. Bosch supplied the ECU (engine control unit) that VW programmed to recognise when an emissions test was taking place.

“Bosch played a crucial role in the fraudulent enterprise, and profited handsomely from it,” state the papers from law firm Lieff Cabraser Heimann & Bernstein. “It is inconceivable that Bosch did not know that the software it was responsible for defining, developing, testing, maintaining and delivering contained an illegal defeat device.”

Bosch has previously said it is not responsible for how a manufacturer integrates its components into vehicles.

The allegations refer to information from confidential documents supplied by Volkswagen, the details of which remain largely out of the public domain. However, they include records of communications between Bosch, VW and US regulators. The court filing states that one 2011 email to the California Air Resources Board shows “Bosch’s deep understanding of what regulators allowed and would not allow, and what Bosch did to help VW obtain approval”.

Bosch is one of the world’s largest automotive suppliers to car manufacturers. It has previously said that it will cooperate fully with investigations and has set aside £560 million (around Rs 4,900 crore) for legal expenses.

The dieselgate scandal, which broke in September last year, centres around Volkswagen’s deliberate cheating of emissions testing systems in the US, which affected some 11 million vehicles worldwide. It has resulted in lawsuits and legal action in countries around the world, including Germany and South Korea.

Last month, VW agreed a $15.3 billion (over Rs 99,000 crore) buyback and compensation deal with US authorities, which will result in American customers being compensated. However, no compensation is being offered in Europe.