Volkswagen is facing strengthening internal resistance to the sale of its Ducati motorcycle unit, threatening to derail the deal even as the process moves forward with a short list of five bidders set to submit binding offers by the end of September, according to Bloomberg sources.

Persistent opposition from VW's powerful labour unions and doubts within the carmaker's Porsche and Piech owner families about the disposal will test management's resolve to selling a non-core asset to reduce corporate complexity, the sources said.

Executives involved have been puzzled by mixed signals, since Audi, which owns Ducati, only started exploring a deal after parent company managers urged the division to do so, the sources said.

Spokesmen for Volkswagen, Audi and the families' holding company declined to comment to Bloomberg.

A spokesman for VW's works council told Bloomberg labour leaders continue to oppose asset sales in general. Bloomberg in turn noted worker representatives comprise half of Volkswagen's 20-member supervisory board and can veto strategic decisions.

Initial offers valued Ducati at as much as EUR1.5bn (US$1.8bn).