Volkswagen warned over test cheating previously

A Volkswagen engineer warned the German car manufacturing giant about cheating over its emission tests as early as 2011, media reported on Sunday.

German daily "Frankfurter Allgemeine Sonntagszeitung" said the warning emerged during Volkswagen's current investigation into the scandal, BBC reported.

Separately, "Bild am Sonntag" daily said the internal inquiry had found that parts supplier Bosch had warned Volkswagen not to use its software illegally.

Citing unidentified sources, Bild said Bosch had warned Volkswagen as early as 2007 that its software should only be used in company tests and not for normal driving.

However, the company has refused to comment over the reports of the two dailies.

Volkswagen has been engulfed in a scandal where it emerged that some of its diesel cars had been fitted with devices that could detect when they were being tested, concealing the real level of pollutants being emitted by them when on the road.

Half of the company's sales in Europe - Volkswagen's biggest market - are for diesel cars.

VW shares plunged around 30 percent after the scandal broke.