Referee Mark Geiger has "strongly refuted" claims he asked for the shirt of Portugal captain Cristiano Ronaldo at half-time of the clash with Morocco on Wednesday.
The allegation was made by Morocco midfielder Nordin Amrabat after a game in which American official Geiger made a number of calls that were questioned by the north Africans.
Amrabat told Dutch television station NOS: "I do not know what he is used to but he was very impressed with Cristiano Ronaldo and I hear from [Portugal defender] Pepe that he asked Ronaldo in the first half if he could have his shirt.
"What are we talking about? At the World Cup? It’s not a circus here."
FIFA responded by backing Geiger and expressing their disappointment at Amrabat's outburst.
In a statement it said: "It is with regret and disappointment that, following yesterday's Group B game between Portugal and Morocco, FIFA has learnt of reports in the media concerning the referee Mark Geiger.
"It has been claimed that Mr Geiger asked for the shirt of the captain of Portugal during the half-time interval.
"Mr Geiger strongly refutes those claims and categorically states that such a request was not made.
"FIFA unequivocally condemns the allegations supposedly made by a member of the Moroccan team.
"FIFA referees are under clear instructions with regard to their behaviour and relationship with the teams at the World Cup in Russia and it can be confirmed that Mr Geiger has acted in an exemplary and professional manner as an appointed match official.
"FIFA would like to remind teams of their duty to respect all principles of Fair Play."
Morocco coach Herve Renard singled out Geiger in his post-match press conference, although stopped short of criticising him directly.
However, it was clear the Frenchman thought Ronaldo's fourth-minute header, which turned out to be the only goal, should have been ruled out while Geiger also turned away a number of confident penalty appeals from Morocco.
Renard said: "I'd like you [the media] to be honest. Have a good look at the corner when Portugal scored and have a good look at what their number three is doing and write the truth.
"It's not up to me to comment on refereeing, I would ask you to have a look at the replays, the slow-motion clips, because a foul should have been seen and there should have been a free-kick.
"It was missed and Ronaldo appears. If I criticise referees I will be punished so the only thing you should write is we are very proud of our performance."
The defeat meant Morocco became the first team to be eliminated from World Cup 2018.
|Ibrahimovic praises Pogba after World Cup final display|
|Wimbledon diary: Avengers too late for an Infinity War as Djokovic triumphs|
|Deschamp-agne – France´s 2018 vintage interrupt news conference to drench coach|
|I may have to dust my boots off! - Pele threatened by Mbappe exploits|
|WATCH: Mendy and Pogba dab with president Macron after World Cup win|
|Woods embracing ´different´ Carnoustie challenge ahead of Open return|
|Argentina dismiss Sampaoli after chaotic World Cup|
|France celebrates World Cup triumph|
|Djokovic made ´improvised Wimbledon trophies´ as a boy|
|Stone goes agonisingly close to 59 as Pepperell & Dantorp also make Open|
|Outcast Benzema delights in France win|
|Final a triumph in itself given Croatia´s off-field turmoil|
|Anderson laments Wimbledon final nerves|
|Deschamps joins elite club, Mbappe second to Pele - the best stats from the World Cup final|
|Belgium´s Courtois wins World Cup Golden Glove|
|Collarbone fracture confirmed for ´devastated´ Porte|
|World Cup final player ratings: Griezmann, Mbappe inspire France as Croatia fall short|
|World Cup winner Giroud ends tournament without shot on target|
|Modric wins World Cup Golden Ball|
|Mbappe wins World Cup Young Player award|
|Deschamps redeemed as France win World Cup final for the ages|
|France 4 Croatia 2: Twitter Report|
|Tomlin optimistic over Bell contract|
|Kane wins World Cup Golden Boot|
|Ajax keen on Blind deal - Van der Sar|
|France 4 Croatia 2: Pogba and Mbappe seal thrilling World Cup triumph|
|Anderson would have played another 21 hours to reach Wimbledon final|
|Djokovic delighted to crown ´comeback´ with Wimbledon glory|
|World Cup final interrupted by pitch invaders - Pussy Riot claim responsibility|
|Djokovic´s grass-court U-turn pays ultimate dividends with Wimbledon title|
|Mandzukic makes World Cup history with early own goal|
|Dominant Djokovic beats Anderson to regain Wimbledon title|
|Everybody said I´m done - Degenkolb revels in first Tour victory|
|´Big party´ for Marquez after Sachsenring success|
|Hazard enjoys Belgium´s welcome home|
|Ex-AC Milan president Berlusconi conflicted over Ronaldo to Juventus|
|Spain did not contact Martinez over head coach vacancy|
|Maybe the VAR was watching The Simpsons! - RSL coach Petke slams MLS officials|
|Cubs´ Darvish hopes to return by next weekend|
|France and Croatia name unchanged sides for World Cup final|
|Marquez maintains Sachsenring dominance|
|Courtois claims World Cup has boosted his value|
|Buffon beaten on difficult PSG debut|
|Vince called up for Headingley decider|
|Farah inspires Tigers win over Dragons, Warriors and Roosters triumph|
|Crash forces Porte out of Tour for second year running|
|Ronaldo considered Napoli move, says De Laurentiis|
|Mafi arrested for allegedly assaulting Rebels team-mate|
|Everybody is welcome - Croatia pitch for neutral support in World Cup final|
|West Ham seal club-record swoop for Felipe Anderson|
|Rooney applauds England for capturing fans´ imagination|
|Krajicek urges Anderson to go for broke in Wimbledon final|
|Wimbledon 2018: Djokovic v Anderson in Opta numbers|
|Kohli defends Dhoni after Lord´s boos|
|Wimbledon finalist Djokovic can win the next six majors, claims Krajicek|
|We´re ´blessed´ to watch Mbappe, says Kaka|
|Bogaerts hits walk-off grand slam to deliver Red Sox win over Blue Jays|
|Cardinals dismiss Matheny, name Shildt interim manager|
|Pacquiao knocks out Matthysse to win welterweight title|
|Didulica expects ´cagey´, tight World Cup final|
|MLS Review: Rooney impresses on debut, Galaxy leave it late|
|Truex dominates to win Quaker State 400 again|
|Kim in control at John Deere Classic|
|Braves place closer Vizcaino back on DL, manager concerned about injury|
|Long on Eagles having Wentz, Foles: It´s really a dream|
|Effenberg: I´d prefer Mandzukic over Neymar|
|Courtois open to Chelsea stay amid Madrid links|
|Rays´ Ramos won´t play All-Star Game due to hamstring injury|
|Gary Sanchez injury update: Yankees catcher on track to return after All-Star break|
|Windies crush Bangladesh again to seal series win|