The Volkswagen diesel emissions scandal has deepened after US authorities accused the carmaker of installing defeat devices into luxury sports cars including Porsches.

The Environmental Protection Agency (EPA), which uncovered the initial emissions rigging at VW, claims the carmaker installed defeat devices in VW, Audi and Porsche vehicles with three-litre engines in models with dates ranging from 2014 to 2016

This marks the first time that Porsche, which is owned by VW, has been dragged into the scandal. It is troubling for the new chief executive of VW, Matthias Müller, because he ran Porsche before becoming boss of the group.

The EPA has made the allegations after conducting further tests on diesel vehicles in the US since VW admitted in September it had used defeat devices to cheat emissions tests.

The new allegations include the 2015 Porsche Cayenne as well as the 2014 VW Touareg and the 2016 Audi A6 Quattro, A7 Quattro, A8, A8L, and Q5. In total, it involves 10,000 vehicles in the US.

In a statement VW denied it had fitted any devices on the vehicles. The statement 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 clarify this matter in its entirety.”

VW has already admitted fitting a defeat device to 11m vehicles worldwide, but this related to cars with smaller engines and did not include any Porsche cars or sports utility vehicles (SUVs).

Cynthia Giles, assistant administrator for the office for EPA’s enforcement and compliance assurance, said: “VW has once again failed its obligation to comply with the law that protects clean air for all Americans. 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.”

The EPA has issued a second notice of violation (NOV) of the Clean Air Act to VW as a result of its findings. VW faces fines of up to $37,500 per vehicle, which means an extra $375m (£243m) could be added to its penalities if it is found guilty. The company already faces a potential $18bn fine for the initial recall by the EPA in September of 482,000 VW and Audi cars.

Richard Corey, executive officer of the California Air Resources Board, said: “On 25 September, the California Air Resources Board sent letters to all manufacturers letting them know we would be screening vehicles for potential defeat devices. Since then ARB, EPA and Environment Canada have continued test programmes on additional diesel-powered passenger cars and SUVs. These tests have raised serious concerns about the presence of defeat devices on additional VW, Audi and Porsche vehicles.

“Today we are requiring VW Group to address these issues. This is a very serious public health matter. ARB and EPA will continue to conduct a rigorous investigation that includes testing more vehicles until all of the facts are out in the open.”

The carmaker has put aside €6.7bn (£4.4bn) to meet the costs of recalling the 11m vehicles, but also faces the threat of fines and legal action from shareholders and customers.

The company has hired the accountancy firm Deloitte and the law firm Jones Day to investigate who fitted the device into its vehicles. It is understood that the carmaker believes a group of between 10 and 20 employees were at the heart of the scandal.

Martin Winterkorn stepped down as chief executive of VW as a result of the scandal. His replacement, Müller, last week vowed to be “ruthless in punishing those involved”, adding: “We are leaving no stone unturned to find out what exactly happened and to make sure nothing like this ever happens again.”