By Ilona Wissenbach and Edward Taylor

FRANKFURT (Reuters) - Volkswagen <VOWG_p.DE> said it was weighing steps against ex-Chairman Ferdinand Piech after media reports said he had informed key supervisory board members about potential diesel cheating six months before the scandal became public.

While the allegations about whether Piech had informed the company's top directors as early as March 2015 could not immediately be verified, Volkswagen's strong rejection of the claims shows a deepening rift with the ousted chairman.

Ferdinand Piech or his representatives could not be reached for comment.

"The supervisory board of Volkswagen AG emphatically repudiates the assertions made by Ferdinand Piech as reported recently in the media," VW's supervisory board said in a statement on Wednesday.

"The board of management will carefully weigh the possibility of measures and claims against Mr Piech," it said, adding that VW would not comment on ongoing investigations as a matter of principle.

Labour representatives Bernd Osterloh and Berthold Huber, both members of the supervisory board's steering committee at the time, called for Volkswagen's management to consider legal steps against Piech.

Huber, who took over as interim chairman after Piech was ousted, has since left the board.

"The allegations are untrue. Had Dr Piech informed us, we may have been able to spare the company and its workforce from substantial harm. We now expect the management board to thoroughly evaluate whether steps need to be taken against Piech," Osterloh and Huber said in a joint statement issued late on Wednesday.

Huber told Reuters: "I can swear in any court in the world that Piech did not talk to me about the matter."

Bild am Sonntag said Piech had raised the issue with then-Chief Executive Martin Winterkorn and members of the supervisory board steering committee in March 2015 after getting a tip-off from an Israeli security firm.

Winterkorn had assured him that everything was under control, Bild am Sonntag reported.

Winterkorn was not immediately available for comment.

Volkswagen admitted in September 2015 to having installed software to cheat diesel-emission tests in the United States, causing a collapse in its share price, Winterkorn's resignation and tens of billions of euros in fines and legal costs.

Piech had said the previous April he was "distancing himself" from Winterkorn, without elaborating, sparking a power struggle that saw senior VW figures rallying around Winterkorn and forced family patriarch Piech to step down.

(Additional reporting by Andreas Cremer and Jan Schwartz; Writing by Edward Taylor; Editing by Christoph Steitz and Georgina Prodhan)