ESPN sources have confirmed that Chelsea's official complaint against referee Mark Clattenburg, which is being investigated by the Football Association, is that he racially abused Jon Obi Mikel and xenophobically insulted Juan Mata.
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The FA announced on Monday that it would be conducting an official investigation. It does not deny that the investigation, which will include gathering video evidence and talking to all four officials at the game, centres around two major incidents.
Chelsea's complaint is that Mikel was the victim of a racist comment, while Spain midfielder Mata was allegedly called a "Spanish t**t" by Clattenburg.
ESPN can reveal that both incidents occurred on the field of play during the Sunday afternoon Stamford Bridge clash with Manchester United and not, as had been reported, in the tunnel.
Chelsea spoke to both players about the incidents at length before lodging their complaint with the FA, which is now certain to contact Sky and seek footage from the broadcaster's 20 cameras at Stamford Bridge.
In an earlier statement confirming its investigation into Chelsea's allegations of "inappropriate language" being used by Clattenburg, the FA said: "The FA has begun an investigation relating to allegations made following Sunday's fixture at Stamford Bridge between Chelsea and Manchester United (Sunday 28 October 2012). The FA will make no further comment at this time."
Gosforth official Clattenburg had a controversial game, sending off two Chelsea players and allowing Javier Hernandez's winning goal to stand despite the striker being in an offside position.
The Professional Game Match Officials organisation later confirmed that Clattenburg will not officiate a match this weekend stating, "that with any football match the focus should not be on the officials but on the players and the game itself.
"Mark Clattenburg is one of the elite referees in world football and, in these circumstances, the intense level of scrutiny would detract from the match and be unfair to the clubs and the supporters of both sides.''
The head of the Professional Footballers' Association, Gordon Taylor, meanwhile, has urged the FA to deal with the accusations as quickly and fairly as possible.
Taylor is wary of the fact that the most recent row concerning racism in football - that involving Blues captain John Terry - dragged on for almost a year before he was banned by the FA, and he does not want this case to go on for such a long period of time.
"We've got to learn the lessons of the last 12 months,'' Taylor said. "This is extremely serious for the referee involved so that's why it's important that it is dealt with in the most thorough manner and as quickly, efficiently, and transparently as possible.''
The police could also yet become involved after the lawyer behind the mooted black players' breakaway union, Peter Herbert, made a written complaint to the Metropolitan Police Service (MPS).
He wrote: "The Society of Black Lawyers wishes to record the incident that took place at Chelsea FC as a racial incident according to ACPO [Association of Chief Police Officers] guidelines.
"Our information is that racist remarks were directed at John Mikel Obi and at Juan Mata. If so, that is wholly unacceptable in any circumstances. Although this matter may be investigated by the FA it is appropriate that the MPS independently see if a racially-aggravated offence has occurred.''
The police confirmed they had received what they labelled a "complaint'' and were considering it.
They said in a statement: "On the afternoon of 29 October, the Metropolitan Police received a complaint relating to the Chelsea v Man Utd match on 28 October. This is now being considered.
"Officers from Hammersmith & Fulham borough have been in contact with Chelsea Football Club today regarding the widely-reported club referral of the referee's behaviour to the Football Association.
"At this time, the Metropolitan Police has not received any complaint from either Chelsea Football Club or the Football Association. We will work in partnership with Chelsea Football Club and the Football Association in order to consider any allegation that is made in relation to the reported events.''
Prospect, the referees union, responded on Monday evening by saying: "We are committed to helping to eradicate racism in football and in society generally. In the context of that commitment, Prospect is offering full support to Mark Clattenburg in relation to the allegations made against him."
Information from the Press Association was used in this report