Real Madrid president Florentino Perez claims Inter attempted to sign Luka Modric without paying for the star midfielder.
Modric – who was crowned UEFA Player of the Year for 2017-18 on Thursday – emerged as Inter's primary transfer target following the World Cup in Russia but the Italian side have been unable to lure the 32-year-old midfielder to Milan.
Madrid sent an official complaint to FIFA, alleging Inter contacted the player illegally over a potential move while the Serie A outfit confirmed legal action against La Liga president Javier Tebas, who criticised the club's pursuit of Modric.