Insiders say Volkswagen's staff and suppliers knew about software designed to thwart emissions tests years ago.

As the Volkswagen scandal deepens, two German newspapers have reported that the company’s executive board received an internal report showing VW technicians warned about illegal emissions practices back in 2011.

It comes after countries around the world launched their own investigations into revelations that VW has systematically installed software in diesel engines that can hide their emissions of toxic gases.

The software disables emissions-reduction measures while the car is in normal operation, but could sense when an emissions test was being performed, and turned them back on.

Engineers say the true emissions of the cars could be dozens of times higher than the tests would show.

A report in the Frankfurter Allgemeine Sonntagszeitung has revealed that VW's supervisory board knew about the issue at least four years ago, while the Bild am Sonntag newspaper has published a report claiming a VW internal probe turned up a letter from parts supplier Bosch in 2007 that also warned Bosch-supplied software technology was being inappropriately modified.

The paper could not cite a source for its report, and a spokesperson for Bosch said internal dealings with VW were confidential.

Some parts of the German automobile and political sectors are distancing themselves from the scandal, with environment minister Barbara Hendricks saying it must not be allowed to tarnish “the ‘made in Germany’ brand”.

“If a global player from Germany violates environment protection rules that blatantly, this casts a shadow on the environment pledges of German companies,” she told the Handelsblatt newspaper.

The European Union is reportedly working on new, stricter emissions tests that will focus more on normal road conditions instead of lab results alone.

It appears that VW’s Italian unit has told dealers to stop selling the cars built with the affected engines.

“As a precautionary measure, we ask that you suspend immediately the sale, registration and delivery only of vehicles carrying the Euro 5, EA 189 motor,” newspaper Corriera della Sera quoted Massimo Nordio, chief executive of Volkswagen's Italian unit, as having written in a letter to dealers.