Chelsea announced on Wednesday that they have lodged a formal complaint with the Football Association over allegations referee Mark Clattenburg used "inappropriate language" towards John Mikel Obi.
Chelsea allege that Clattenburg abused Nigerian midfielder Mikel during the European champions' stormy 3-2 loss to Manchester United in the Premier League at Stamford Bridge on Sunday.
"Chelsea Football Club today lodged a formal complaint with The Football Association regarding inappropriate language directed at John Mikel Obi by referee Mark Clattenburg," read a statement from Chelsea.
"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.
"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."
Chelsea added that the decision to inform the match delegate of Clattenburg's alleged remarks was "not a decision the club took lightly".
Reports in the British media have suggested that Chelsea believe the language used by Clattenburg towards Mikel was of a racial nature.
The club's initial complaint claimed Clattenburg had used inappropriate language towards two players, but the allegation regarding the other player -- thought to be Spanish midfielder Juan Mata -- has been dropped.
"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 said.
The FA announced on Monday that they have launched an investigation into the incident, while the Metropolitan Police said on Tuesday that they will also conduct an inquiry.
"We will co-operate fully with the FA and the police as they each investigate the matter," continued the statement from Chelsea.
"As events on Sunday are now subject to those investigations, we are unable to comment further until those processes are complete."
The game in question saw two Chelsea players sent off by Clattenburg, while United's 75th-minute winning goal was allowed to stand despite being scored from an offside position.
Clattenburg, 37, reportedly denies the allegations and has vowed to co-operate fully with any investigations.