A union has called a strike at Fiat Chrysler's Melfi plant in Italy to protest against the huge sums of money spent by Juventus soccer club to sign up Cristiano Ronaldo.

Exor, the investment holding of Italy's Agnelli family, owns nearly 30 percent of Fiat Chrysler (FCA) and 64 percent of Juventus.

FCA and Juventus are completely separate entities, each managing their own finances and operations.

Italian champions Juventus agreed on Tuesday to pay Real Madrid €100 million to secure the Portuguese striker for four years. Ronaldo has been world player of the year five times.

"It is unacceptable that while the (owners) ask workers of FCA ... for huge economic sacrifices for years, the same decide to spend hundreds of millions of euros for the purchase of a player," the USB union said in an emailed statement.

Thousands of FCA plant workers across Italy have been on state-sponsored temporary layoff schemes for years due to lack of new models.