Shares in Volkswagen tumbled on the Frankfurt stock exchange Tuesday amid new accusations in the ever-widening emissions cheating scandal, but the German auto giant adamantly denied the new charges.

The inclusion of Porsche vehicles among those alleged to contain devices designed to cheat pollution controls could trip up VW's new chief executive, Matthias Mueller, who was drafted in from the luxury sports car unit.

VW shares skidded 5.1 percent lower at the start of trade to a daily low of 107.00 euros after US regulators accused it late Monday of fitting illegal "defeat devices" into its larger 3.0 litre diesel engines.

The share later pared losses to close down 1.5 percent at 111 euros while Frankfurt's DAX 30 index was flat overall.

In an affair that has rocked the automobile sector around the world since it broke in September, the carmaker has already admitted using the software, which skews the results of pollution tests, in smaller 2.0 litre diesels in some 11 million cars worldwide.

But the US Environmental Protection Agency said late Monday it had discovered that various six-cylinder 3.0 litre diesel VW Touareg, Porsche Cayenne and Audis were also rigged with the software.

"We have clear evidence of these additional violations," said Cynthia Giles, an official with the EPA's Enforcement and Compliance Assurance office.

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

- VW denies new allegations -

VW, the world's number two carmaker by sales, denied the new charges.

"Volkswagen AG wishes to emphasise that no software has been installed in the 3-litre V6 diesel power units to alter emissions characteristics in a forbidden manner," it said in a statement.

"Volkswagen will cooperate fully with the EPA (to) clarify this matter in its entirety."

Porsche similarly denied the allegations, insisting that "all of our information was that the Porsche Cayenne Diesel is fully compliant".

Audi, another of VW's high-end brands, insisted that the software installed in its engines were "in line with the law".

It argued that the so-called Auxiliary Emission Control Devices (AECD) fitted into its engines were not designed to cheat pollution tests, but maximise the engine's performance under different driving conditions.

Carmakers were allowed to fit AECDs in cars in the United States as long as they kept the authorities informed once a year, a spokesman explained.

In terms of image, the scandal is turning into a veritable car crash for a company long seen an emblem of Germany's industrial might.

Already one chief executive of the company, Martin Winterkorn, has lost his job, and shares in the giant have shed nearly one-third of their value since the scandal erupted.

The new US notice of violation for the larger-engined cars, could weigh on Winterkorn's replacement Mueller, who was elevated from running VW's Porsche subsidiary. At the time of his promotion, Porsche vehicles were not known to have the defeat devices.

Moreover, the first notice of violation on September 18, which triggered the scandal, made clear that from the EPA alone, the company was facing a potential $18 billion in fines, based on the maximum penalty per vehicle and the half-million US-sold cars involved.

There are also a number of owner lawsuits against the company, and it could be hit with fines in other countries and regions as well.

- Financial losses -

Chancellor Angela Merkel, speaking to the BDI industry federation in Berlin, was adamant that the affair would not tarnish Germany's pristine engineering image.

"But we must insist on transparency and a rapid investigation," she said. "And my assumption is that those involved will do that."

The US House of Representatives' Energy and Commerce Committee, which has opened a probe into the emissions scandal, said the new allegations meant "it's time for Volkswagen to fully come clean".

"The EPA expanding its investigation prompts questions regarding the prevalence of the emissions cheating and how it went undetected for so long," committee leaders said in a statement.

The scandal is clearly taking a toll on Volkswagen's finances.

Last week, the company booked its first quarterly loss in more than 15 years as it set aside 6.7 billion euros ($7.4 billion) to cover the initial costs of the scandal.

Chief financial officer Frank Witter warned of further "considerable financial charges" from the emissions case.

It was "far too early to calculate the cost of any legal measures," Witter said. "The financial burden will be enormous, but manageable."

The scandal appears to be taking a toll on Volkswagen's sales in the United States, which stalled in October while other major US automakers enjoyed double-digit gains in what was their best month since 2001.