Chelsea have lodged a formal complaint with the Football Association over "inappropriate language" they claim Mark Clattenburg used against John Obi Mikel in their acrimonious defeat to Manchester United.
But the Blues decided not to pursue an allegation referee Clattenburg also used words understood to have been interpreted as racist towards Juan Mata during Sunday's game following an internal investigation carried out by external legal counsel.
Press Association Sport understands Chelsea submitted signed witness statements from a number of players and staff to the FA, who had already begun their own probe into the matter.
The police are also investigating the allegations against Clattenburg, who it is understood categorically denies any wrongdoing.
Chelsea said in a statement: "Chelsea Football Club today lodged a formal complaint with The Football Association regarding inappropriate language directed at John Mikel Obi by referee Mark Clattenburg.
"This is a result of a thorough investigation carried out by the club, and led by outside legal counsel, of all information available to us including interviews with a number of players and staff."
The statement continued: "Having completed that process, we have now followed the correct protocol and lodged a formal complaint with the FA regarding John Mikel Obi.
"There was not sufficient evidence to support a second claim to the FA with regard to the alleged verbal abuse of another first-team player. We will cooperate fully with the FA and the police as they each investigate the matter."
The FA did not respond to the development as they continued their own probe, while the Metropolitan Police were not immediately available for comment.
It is thought Clattenburg is yet to be interviewed by either organisation but he and his fellow referees were said to have been shocked an angered by the claims against him. The 37-year-old had already vowed to co-operate with any inquiry.