Volkswagen Group has denied claims that 3.0-litre V6 diesels in VW's, Audis and Porsches have been fitted with software to defeat emissions test.
The US's Environmental Protection Agency (EPA) has issued a notice of violation (NOV) of the Clean Air Act (CAA) to Volkswagen, Audi and Porsche.
The notice of violation affects 10,000 vehicles sold in the US, including Volkswagen Touareg, Porsche Cayenne, and Audi Q5 models.
"VW has once again failed its obligation to comply with the law that protects clean air for all Americans," claimed Cynthia Giles, assistant administrator for the Office for EPA's Enforcement and Compliance Assurance.
"All companies should be playing by the same rules. EPA, with our state, and federal partners, will continue to investigate these serious matters, to secure the benefits of the Clean Air Act, ensure a level playing field for responsible businesses, and to ensure consumers get the environmental performance they expect."
Volkswagen Group issued a statement today denying the claim, which said: "Volkswagen AG wishes to emphasise that no software has been installed in the 3-liter V6 diesel power units to alter emissions characteristics in a forbidden manner.
"Volkswagen will cooperate fully with the EPA to clarify this matter in its entirety."