Disgraced former Volkswagen personnel chief Peter Hartz has defended the company's ex-boss Ferdinand Piech against charges that he knew of corrupt practices used to ease labour relations.

According to Agence France Press (AFP), Hartz told a court in Braunschweig on Thursday that he only told Piech in "an informal manner" in 1994 of former VW works committee head Klaus Volkert's demands to be paid better.

"The details were not mentioned," Hartz, who was convicted in connection with the bribery scandal at the beginning of the year, was quoted as saying.

Hartz received a two-year suspended sentence and was ordered to pay a large fine after acknowledging he had approved around EUR2.6m ($US3.7m) in payments to Volkert, who was also a former trade union leader.

He was Thursday testifying in the third trial to flow from the massive corruption scandal at Europe's biggest carmaker in which VW executives used a slush fund to pay for the services of prostitutes and for exotic trips as favours designed to ensure peaceful labour relations at the company, AFP noted.

Volkert and another former head of VW personnel, Hans-Joachim Gebauer, face charges of inciting breach of trust, and breach of trust, respectively, the news agency said.

AFP added that German judicial officials also want to determine the role of top VW directors and to find out if former boss Piech, now head of the company's supervisory committee, had been informed of the practices.

Piech has steadfastly denied any knowledge, but news magazine Der Spiegel has published a letter that a former aide is said to have sent to him that raised the question of the trips, the report said.

A VW spokesman told AFP: "We have seen the letter and we doubt it is authentic."

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