Munir, currently on a season-long loan at Deportivo Alaves, was born and raised in Spain but has a Moroccan father.
The 22-year-old made his debut for the Spanish senior side under Vicente del Bosque in 2014 in La Roja's 5-1 victory over Macedonia during their Euro 2016 qualifying campaign.
Munir replaced Atletico Madrid's Koke with 13 minutes remaining of the tie in what remains his only appearance for the country.
The forward has made no secret about his desire to represent Morocco, however, who have been drawn in the same World Cup group as Spain ahead of this summer's tournament.
FIFA rules state a player cannot switch allegiance once they have represented a nation in a competitive international game, though, even if the individual holds dual nationality.
CAS has now revealed in a statement that Munir had appealed alongside the Moroccan Football Federation (FMF) after FIFA turned down his request on March 13.
"The Appellants have requested that the Arbitral Award be issued by mid-May in order for the Player’s sporting nationality to be decided prior to the commencement of the FIFA 2018 World Cup.
"Munir El Haddadi and the Moroccan Football Federation are challenging the decision issued on 13 March 2018 by the FIFA Players' Status Committee in which their request to have the sporting nationality of Munir El Haddadi changed from Spain to Morocco was rejected.
"A CAS arbitration procedure is in progress. The Appellants have requested that the Arbitral Award be issued by mid-May in order for the player's sporting nationality to be decided prior to the commencement of the FIFA 2018 World Cup."
Munir has scored 11 goals in 31 appearances for Alaves so far this season.