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.
"Chelsea Football Club today lodged a formal complaint with The Football Association regarding inappropriate language directed at John Mikel Obi by referee Mark Clattenburg," Chelsea said in a statement.
"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."
The matter is already the subject of FA and police investigations and Chelsea's statement added: "We will cooperate fully with the FA and the police as they each investigate the matter.
"As events on Sunday are now subject to those investigations, we are unable to comment further until those processes are complete."
The FA refused to comment on the development as they continued their probe, while the Metropolitan Police were not immediately available for comment.
Press Association Sport understands Clattenburg categorically denies any wrongdoing. He and his fellow referees were said to have been shocked an angered by the claims. The 37-year-old has already vowed to co-operate with any inquiry.