Volkswagen is Europe's biggest carmaker

The trial of Volkswagen's former works council chief Klaus Volkert and his management liaison, Klaus-Joachim Gebauer resumed on Monday.

The case centres on claims that VW managers and union leaders used company funds to pay for perks

Mr Piech, now a non-executive chairman at VW has denied any wrongdoing.

Works' council boss Volkert has denied the main charges against him.

Piech is not under investigation at this stage, chief prosecutor Ralf Tacke said.

But Tacke asked the court for permission to interview three more witnesses to find out what Piech knew about the bribery scandal.

"Should concrete evidence emerge from the questioning of witnesses then a preliminary investigation could be considered," Tacke said.

Illegal payments

VW's former head of personnel Peter Hartz got a two-year suspended prison sentence in January and was fined 576,000 euros (�413,349; $855,478) for his role in the scandal, which saw him sanction illegal payments to members of the firm's works council.

Works councils are made up of workers and management and have a strong decision-making role within major companies.

Some have argued that the system is a breeding ground for corruption.