Referee Mark Clattenburg is facing allegations he used racist language towards a Chelsea footballer, the English players' union boss confirmed on Tuesday as police and the Football Association continued their investigations.
The issue is threatening to plunge English soccer back into another racism crisis, just as Chelsea captain John Terry is serving a four-match ban for racially abusing Queens Park Rangers defender Anton Ferdinand last year.
Chelsea have made a formal complaint that Clattenburg used "inappropriate language" towards two of their players in Sunday's 3-2 home league defeat by Manchester United but have not named names or gone into detail.
Professional Footballers' Association boss Gordon Taylor said Nigerian midfielder John Obi Mikel and Spanish winger Juan Mata were the players involved, confirming widespread media reports.
"One of the comments was of a racial type of language definitely," he added to Sky Sports News.
"It is of a racist nature."
While Clattenburg has made no public comment on the case, Sky reported he has strongly denied using any inappropriate language and is "mortified" by the allegations.
AS newspaper in Spain reported, from unnamed sources, that the referee called Mata "an idiot Spaniard" while British media have instead said he is alleged to have used an offensive swear word coupled with the player's nationality.
Clattenburg will not be officiating at any Premier League matches this weekend after the Professional Game Match Officials Board (PGMO), which manages top English referees, said on Monday he was being stood down given the "intense level of scrutiny".
A separate PGMO statement said the issue was "being treated with the utmost seriousness" and Clattenburg will co-operate fully and "welcomes the opportunity for the facts to be established".
Former Premier League referee Jeff Winter said Clattenburg would probably never be able to referee another match if the allegations of racist abuse are true.
"If a match official has used racist language, he won't be getting a four or eight-match ban, it'll probably be the end of his career - but that is indeed if he did say anything," Winter told British media.
Liverpool's Luis Suarez had an eight-match ban last term for racially abusing Manchester United's Patrice Evra.
The Metropolitan Police has confirmed it has now received a complaint about the referee and that it will be working alongside Chelsea to determine what action, if any, will be taken.
The FA said on Monday it had begun investigations into the alleged incidents that occurred during United's win at Stamford Bridge where Clattenburg sent off home players Branislav Ivanovic and Fernando Torres.
United, who were dragged back to 2-2 from 2-0, won the game with an offside goal by Javier Hernandez which was not spotted by the linesman and Clattenburg allowed the goal to stand.
Sunday's defeat was Chelsea's first in the Premier League this season but they remained top of the table on 22 points from their opening nine games - one point clear of United and champions Manchester City.
Chelsea host United again in Wednesday's League Cup fourth-round match and although teams often rest leading players in the competition, the clash will take on extra significance given the controversy surrounding Sunday's Premier League encounter.
Although the club would not name the players Clattenburg is alleged to have used inappropriate language towards or of what nature the remarks were, a spokesman refused to deny that Nigerian Obi Mikel had visited the referee's room after the game.
Mikel was booked for dissent during the second half.
Campaigners have long protested at the amount of bad language emanating from players in England despite officials having the power to send off those who use offensive, insulting or abusive words or gestures.
After Sunday's match, a club spokesman told reporters: "We have lodged a complaint to the Premier League match delegate with regards to inappropriate language used by the referee and directed at two of our players in two separate incidents.
"The match delegate will pass the complaint to the Football Association. We will make no further comment at this time."
An investigation is also being carried by the Metropolitan Police into how a Stamford Bridge steward was injured following a Chelsea crowd surge after Hernandez's winner. Objects were also seen to be thrown on to the pitch from the stands during the fractious match.
Terry is currently halfway through his four-game domestic ban, which started just after the English FA complained about the racist abuse of England Under-21 players in an unrelated incident at a match in Serbia.
The Terry case has blighted the English game for the past year after the former England skipper was also ordered to appear in court accused of a racially aggravated public order offence, of which he was acquitted.
The FA, however, using different standards of proof to guide its verdict, found him guilty and fined him 220,000 pounds ($354,200) as well as banning him. Terry decided not to appeal.