Workers at a Fiat Chrysler plant in Italy are to strike after its main investor decided to pay €112m (£99.2m) to sign footballer Cristiano Ronaldo for Juventus.
Both the football club and the carmaker are controlled by the Agnelli family through their holding company.
For the USB union, the decision means Fiat is missing out on investment.
It said the firm needed to guarantee the future of thousands of people, “rather than enriching only one”.
The union added that it was “unacceptable” that while Fiat Chrysler workers were making “huge economic sacrifices”, millions of euros were being spent on the purchase of a player.
The four-year deal to woo Ronaldo from Real Madrid was announced on Tuesday amid concern that Juventus might have overpaid for the 33-year-old forward.
However, football finance expert Rob Wilson, of Sheffield Hallam University, said Juventus should earn more than enough money from Ronaldo to cover the transfer fee and his wages.
He added: “The marketing leverage that Juventus will be able to create will be significant. Added to that the likelihood that he will strengthen the team, it seems plausible that they will be more successful domestically and qualify routinely for the Champions League. That means more sponsors, more TV money and more prize money.”