Last month's change in the ruling on players switching nations opened the door for the Sevilla attacker to represent the Atlas Lions, despite having featured for Spain's senior team in a competitive match.
He had duly been named in the Morocco squad for this international window -- the first for African nations since the coronavirus pandemic began -- and had been primed to make his international debut in a friendly against Senegal on Friday.
However, a statement released by the Royal Morocco Football Federation (FRMF) on Friday, as seen by ESPN, revealed that FIFA have blocked his request to switch nations because the player represented Spain at U21 level when he was already 21.
"I'm not giving up," El-Haddadi wrote on social media, "and I'll continue working to wear the shirt and represent the Morocco national team."
The federation's statement confirmed that FIFA blocked the forward's switch because of his appearances for Spain's U21 team, not because of his sole competitive appearance for the seniors.
"This rejection is dictated by one of the clauses of the new [FIFA] statue concerning [El-Haddadi's] participation in the under-21 category," the statement said.
"In particular during three matches with the Spanish U21 selection when he was two months older. The decision is not motivated by the participation of El-Haddadi in the Spanish A team."
The forward made a 13-minute cameo for Spain in a 2016 European Championships qualifying match against Macedonia in 2014, which, under FIFA's previous guidelines, had prevented him from representing Morocco.
However, last month, it was determined that players who had represented one nation in a senior competitive international could switch nations providing various criteria were met, including that they did not represent their first nation at the age of 21 or over.
Despite Munir's sole Spain appearance coming when he was 19, he later made appearances for Spain's U21 side in late 2016, following his 21st birthday.
Morocco have confirmed that they will appeal the decision.
"After discussions between the FRMF and FIFA," the statement concluded, "the FRMF have decided to appeal by resorting to the Arbitral Appeal Procedure at the Court of Arbitration for Sport."
Munir, who underwent his initiation ceremony with his new Morocco teammates earlier this week, had been named in the squad alongside former Germany youth international Aymen Barkok and ex-Belgium U21 international Samy Mmaee.
Both players have been cleared by FIFA to represent the country moving forward.