German carmaker Volkswagen (VW) said it had ordered an external investigation of the violation of US emission rules.

The company made the statement after the US Environmental Protection Agency (EPA) and the California Air Resources Board found that the software on VW diesel cars showed false emission data, reported Xinhua news agency.

"The Board of Management takes these findings very seriously. I am deeply sorry that we have broken the trust of our customers and the public. We will cooperate with the responsible agencies, with transparency and urgency, to clearly, openly, and completely establish all the facts of this case," Xinhua quoted Martin Winterkorn, CEO of Volkswagen AG as saying in a statement released on Sunday.

The software installed by Volkswagen in its cars has violated the Clean Air Act, said the EPA in a statement on Friday.

The software called "defeat device" can turn on full emission controls only when the car is undergoing emission tests to make the car meet the legal emission standards.

During normal driving, the car will emit nitrogen oxides at up to 40 times the standard.

"Using a defeat device in cars to evade clean air standards is illegal and a threat to public health," said Cynthia Giles, EPA’s assistant administrator for the Office of Enforcement and Compliance Assurance.

The allegations cover roughly 482,000 diesel passenger cars sold in the US by Volkswagen AG, Audi AG, and Volkswagen Group of America since 2008. The models include Jetta, Beetle, Golf, Passat and Audi A3.

The EPA also said "Volkswagen may be liable for civil penalties", which means the German car maker could face penalties of up to $37,500 per vehicle, the maximum fine for violating the Clean Air Act.