Chelsea tonight 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 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."
"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" - Chelsea FC
"Following Sunday's Premier League game against Manchester United, club board members were quickly made aware, upon entering the home dressing room after the final whistle, that there were allegations of inappropriate language directed at Chelsea FC players by Mr Clattenburg during the game.
"Board members began establishing the details of the allegations, speaking to all relevant players and staff. After those initial interviews, it was clear that the matter had to be reported.
"The correct protocol for doing so was to report the incidents to the match delegate, and the club took immediate steps to inform him as soon as he was available. It was not a decision the club took lightly.
"Since reporting the matter to the delegate, the club and outside legal counsel have conducted further investigations.
"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 tonight's development as they continued their probe, while the Metropolitan Police were not immediately available for comment.
Chelsea refused to expand on the precise nature of their own investigation amid reports neither Mikel nor Mata had heard him abuse them but were rather told of the allegations by team-mates.
Sources have said Clattenburg, his assistants and fourth official were stunned after being confronted by an angry Mikel in the referees' room after Sunday's match.
Clattenburg's colleagues, who were miked up to him throughout the game, were all said to have denied hearing him abuse either player.