America's Environmental Protection Agency (EPA) has claimed that it has found more evidence of the Volkswagen Group running an illegal defeat device on its 3.0-litre diesel engines, saying the scandal has now widened to affect Porsche.

The EPA alleges that VW installed software designed to defeat emissions tests on vehicles with 3.0-litre, six-cylinder diesel engines that were on sale from 2013 to date, and says NoX emissions on the affected vehicles are up to nine times legal limits. It has identified the VW Touareg, Porsche Cayenne and Audi A6, Audi A7, Audi A8, Audi A8L and Audi Q5 as being potentially affected. In total, 10,000 cars in the US have been implicated.

"VW has once again failed its obligation to comply with the law that protects clean air for all Americans," said 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."

A statement from Porsche expressed shock at the claims. The statement read: "We are surprised to learn this information. Until this notice, all of our information was that the Porsche Cayenne Diesel is fully compliant. Porsche Cars North America will cooperate fully with all relevant authorities."

A statement from the VW Group said: "The United States Environmental Protection Agency (EPA) informed Volkswagen Aktiengesellschaft on Monday that vehicles with V6 TDI engines had a software function which had not been adequately described in the application process. Volkswagen AG wishes to emphasise that no software has been installed in the power units to alter emissions characteristics in a forbidden manner. Volkswagen will cooperate fully with the EPA clarify this matter in its entirety."

VW's statement suggests that it believes the software is legal, but was not correctly certified. The use of software to protect engines under prescribed circumstances is legal - although cheating emissions tests is forbidden.

The EPA documentation alleges that the affected cars have "software in the electronic control module of these vehicles that senses when the vehicle is being tested for compliance with EPA emissions standards. When the vehicle senses that it is undergoing a federal emissions test procedure, it operates in a low NOx 'temperature conditioning' mode. Under that mode, the vehicle meets emission standards.

"At exactly one second after the completion of the initial phases of the standard test procedure, the vehicle immediately changes a number of operating parameters that increase NOx emissions and indicates in the software that it is transitioning to 'normal mode' where emissions of NOx increase up to nine times the EPA standard, depending on the vehicle and type of driving conditions.

"In other tests where the vehicle does not experience driving conditions similar to the start of the federal test procedure, the emissions are higher from the start, consistent with 'normal mode.'”

Jim Holder (Autocar UK)