Police, lawyers and the English FA were all preparing their ground on Tuesday to determine whether referee Mark Clattenburg used "inappropriate language" towards two Chelsea players in Sunday's defeat by Manchester United.
Former Premier League referee Jeff Winter said Clattenburg would probably never be able to referee another match if media allegations he racially insulted Chelsea midfielder John Obi Mikel and verbally abused Spaniard Juan Mata are true.
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.
While Clattenburg has made no public comment on the case, Sky Sports reported he has strongly denied using any inappropriate language and is "mortified" by the allegations.
The FA said on Monday it had begun investigations into the alleged incidents that occurred during United's 3-2 win at Stamford Bridge where Clattenburg sent off home players Branislav Ivanovic and Fernando Torres.
United won the game with a goal scored by Javier Hernandez who came back from an offside position before firing in from close range. The infringement was not spotted by the linesman and Clattenburg allowed the goal to stand.
He 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".
Former Premier League referee Winter told British media: "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."
Chelsea captain John Terry is serving a four-match ban for racially abusing Queens Park Rangers defender Anton Ferdinand last year while Liverpool's Luis Suarez had an eight-match ban last term for racially abusing Manchester United's Patrice Evra.
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.
Spaniard Mata has also been identified in the media as the other player insulted by Clattenburg with As newspaper in Spain reporting, from unnamed sources, that the referee called him "an idiot Spaniard".
After the 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 Hernandez's winner. Objects were also seen to be thrown on to the pitch from the stands during the fractious match.
The issue is threatening to plunge English football back into another racism crisis, just as the affair involving Chelsea defender Terry is coming to its end.
Terry is currently halfway through his four-game domestic ban.
That 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 as well as banning him. Terry decided not to appeal.
Despite trailing 2-0 and clawing their way back to 2-2, 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 Manchester United and champions Manchester City.