Police have launched a formal investigation into allegations Mark Clattenburg used "inappropriate language" towards John Obi Mikel and Juan Mata during Chelsea's 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 using comments that are understood to have been interpreted as racist remarks.
Clattenburg completely rejects the allegations against him, which are also the subject of a FA investigation.
"It is appropriate that the MPS independently see if a racially-aggravated offence has occurred" - Met Police
The police probe came less than 24 hours after the FA launched its own investigation, which it must now decide whether to continue or postpone until after potential criminal proceedings are concluded.
The British game's governing body came under fire for the way the John Terry case was allowed to drag out and will be desperate to avoid a similar saga with Clattenburg.
It is understood the referee had yet to be interviewed by the FA on Tuesday but was ready to formally deny any wrongdoing having vowed to co-operate fully with any investigation.
He was said today to be shocked and angered at the allegations made against him and confident of clearing his name.
Should the FA continue with its probe, it is expected to interview Clattenburg, possibly after requesting a written account from him 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 abusing him.
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 via 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 yesterday from officiating in the coming week.
Police became embroiled in a second high-profile football racism case last night after the man behind the mooted black players' breakaway union, Peter Herbert, sent a letter to the Metropolitan Police Service (MPS), which read: "The Society of Black Lawyers wishes to record the incident that took place at Chelsea FC.
"Although this matter may be investigated by the FA, it is appropriate that the MPS independently see if a racially-aggravated offence has occurred."
It was unclear whether Mikel, Mata or Chelsea welcomed police involvement but they could be forgiven for being wary of it in the wake of the Terry case.
Clattenburg, who says he will co-operate fully with any investigation, was yesterday promised the "full support" of referees' union Prospect.
Herbert this afternoon defended his decision to involve the police.
"What we don't want is for it to be swept away under the carpet," he told Sky Sports News.
"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."
Herbert also confirmed he planned to make a formal complaint to the Serbian government over alleged racist abuse suffered by England Under-21 players in the country earlier this month.