Police have launched a formal investigation into allegations referee Mark Clattenburg used "inappropriate language" towards John Obi Mikel and Juan Mata during Chelsea's Barclays Premier League defeat to Manchester United.
The Metropolitan Police confirmed they had acted on what they described as a "complaint" made by the Society of Black Lawyers after the Blues accused referee Clattenburg of making comments that are understood to have been interpreted as racist remarks.
Press Association Sport understands the 37-year-old completely rejects the allegations against him, which are also the subject of a Football Association investigation.
A police statement read: "An investigation has been launched into alleged comments made during a football match between Chelsea FC and Manchester United FC at Stamford Bridge on 28 October 2012.
"This follows on from a complaint received by the Metropolitan Police Service on 29 October.
"Officers from Hammersmith & Fulham borough are in liaison with Chelsea Football Club and the Football Association.
"At this time, the MPS has not received any complaint from either Chelsea Football Club, or the Football Association.
"We continue to 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."
The criminal probe came less than 24 hours after the FA launched its own investigation, which it was continuing on Tuesday.
However, it could yet postpone its investigation if the police request it does so.
The game's governing body came under fire for the way the John Terry case was allowed to drag on and will be desperate to avoid a similar saga.
The Metropolitan Police became embroiled in their second high-profile football racism case in 12 months last night after the man behind the mooted black players' breakaway union, Peter Herbert, wrote to them demanding an investigation.
He defended his decision this afternoon, telling Sky Sports News: "What we don't want is for it to be swept away under the carpet.
"It must be subject to a full and proper investigation.
"It is to lend some seriousness and some weight behind what is happening in football."
Herbert admitted his complaint was based on reports rather than first-hand evidence but added: "We weren't there but we don't need to be there in order to report an incident.
"This appears to have had some cogency and so it needs to be addressed as a matter of urgency.
"I think the Met Police have huge resources, expertise, and I have no doubt that this matter will be resolved and the truth will come out.
"If we've got this completely wrong then, of course, the police will tell us."
But Professional Footballers' Association chief executive Gordon Taylor said: "Involving police or waiting causes a massive festering of the issue, which has continued to cause problems and is not good for the image of the game.
"Football has got to be confident enough to deal with it. I have said that to the House of Commons, the FA Council.
"We need to grasp the nettle and show we are more than capable of dealing with it."
He added: "I'm quite concerned that when this happened with the John Terry and Anton Ferdinand incident, the process got elongated.
"This time, I want football to learn from it and deal with it as transparently as possible.
"In football, the penalties can be severe. In a court of law, the penalty for racial abuse would be a small amount in comparison to what the FA could fine."
Taylor welcomed the FA's refusal to halt its own investigation, citing the International Cricket Council's decision to rule on the Pakistan spot-fixing scandal before it went to court.
It is understood Clattenburg had yet to be interviewed by the FA on Tuesday morning but was ready to formally deny any wrongdoing, having vowed to co-operate fully with any investigation.
He was said today to have been shocked and angered at the allegations made against him and confident of clearing his name.
He is now expected to be spoken to by both police and the FA, possibly after submitting a written account about what took place on Sunday.
Clattenburg has already filed what is known as an 'extraordinary incident report', which is understood mainly to deal with an alleged meeting that took place in the referees' room after full-time.
Sources have told Press Association Sport that Chelsea manager Roberto Di Matteo, assistant manager Eddie Newton and chief executive Ron Gourlay were all present as Mikel angrily accused Clattenburg of having abused him during the match itself.
The referee, his assistants and fourth official are understood to have been stunned by the claims, with Michael McDonough, Simon Long and Michael Jones denying hearing anything of that nature over the officials' microphone link-up.
Clattenburg's fellow professional referees were today thought to be as shocked and angered as their colleague over the allegations against him and were rallying around him.
It was unclear whether Clattenburg would elect to continue refereeing or take a break from the game while any investigation was pending but he accepted being stood down from officiating in the coming week.