FRANKFURT, Germany - Shares in automaker Volkswagen are sliding after U.S. environmental officials said the company equipped more models than previously thought with software that let the cars cheat on diesel emissions tests.

The company's ordinary shares fell 3.0 percent to 109.30 each in midday trading in Europe on Tuesday.

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A scandal over emissions cheating widened on Monday when the Environmental Protection Agency said that Volkswagen installed software on thousands of Audi, Porsche and VW cars with six-cylinder diesel engines that allowed them to emit fewer pollutants during tests than in real-world driving. Volkswagen denied the charge, but faces the prospect of more fines and lost sales.

The previous revelations of cheating involved four-cylinder diesels in smaller cars. Volkswagen has apologized for those cars and hired a law firm to investigate.

The new accusations are the first to involve the company's prestigious Porsche brand, which had been headed by Matthias Mueller before he became CEO of the entire company. Mueller took over from Martin Winterkorn, who resigned because of the scandal.

Volkswagen has set aside 6.7 billion euros ($7.4 billion) to cover the costs of recalling the cars. Analysts say the actual losses, including a potential drop in sales, could be several times higher.

U.S. regulators continue to tell owners of all the affected cars they are safe to drive, even as they emit nitrogen oxide, a contributor to smog and respiratory problems, in amounts that exceed EPA standards. The six-cylinder engines emit up to nine times above accepted levels while the four-cylinders emit up to 40 times, the regulators say.

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Volkswagen denied that software was installed on the larger diesels "to change the emissions values in any impermissible way" and vowed to cooperate with the EPA.

German Chancellor Angela Merkel called for a quick resolution in a speech to the Federation of German Industries in Berlin.

"'Made in Germany' remains ... a trademark and a seal of quality, and the events at Volkswagen change nothing about that — I want to stress that again," she said.

"But they also show that we must also insist here on transparency and a quick clearing-up, though I assume those affected will ensure that themselves," she said. Merkel didn't specifically mention the latest EPA allegations.

The new alleged violations cover models including the 2014 VW Touareg, 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8 and Q5. The EPA said a recall is possible, but it wants to make sure whatever fixes Volkswagen proposes are appropriate.