The Frankfurter Allgemeine Sonntagszeitung, citing a source on VW's supervisory board, said the board had received an internal report at its meeting on Friday showing VW technicians had warned about illegal emissions practices in 2011. No explanation was given as to why the matter was not addressed then.

Separately, Bild am Sonntag newspaper said VW's internal probe had turned up a letter from parts supplier Bosch written in 2007 that also warned against the possible illegal use of Bosch-supplied software technology. The paper did not cite a source for its report.