Chelsea have made an official complaint to the English Football Association over claims referee Mark Clattenburg used 'inappropriate language' while sending off two of their players in the 3-2 defeat to Manchester United.
The English FA is expected to receive the match delegate's report featuring the complaint over two comments Clattenburg allegedly made towards two of Roberto Di Matteo's players.
A club spokesman said: "We have lodged a complaint to the Premier League match delegate with regards to inappropriate language used by the referee and directed at two of our players in two separate incidents in today's match.
"The match delegate will pass the complaint to the Football Association.
"We will make no further comment at this time."
Clattenburg last night vowed to cooperate with any investigation.
Professional Game Match Officials (PGMO), the body which represents Premier League referees, released a statement which read: "PGMO is aware of the allegations and they are being treated with the utmost seriousness.
"Mark will cooperate fully and welcomes the opportunity for the facts to be established.
"No further comment will be made until this matter has been properly investigated."
All four officials - the referee, his two assistants and the fourth official - wear microphones and ear pieces that allow them to hear what each other is saying throughout the match. What is said between them is not recorded.
The revelation that Chelsea had made the complaint rounded off a dramatic day which saw the Blues lose their unbeaten top-flight record after Javier Hernandez struck the winning goal 15 minutes from time, sparking angry scenes from some home supporters after Hernandez and Rio Ferdinand celebrated in their direction.
Chelsea said they were also looking into reports that missiles were thrown after a steward was injured.
The steward needed 10 minutes' treatment from paramedics after the incident and was in clear discomfort as he lay slumped against an advertising board.
He was later taken to hospital for treatment.
"We are looking at reports of objects that may have been thrown or incidents around the steward taking a fall," a Chelsea spokesman said after the match.
"We are investigating all the incidents around that time.
"My information so far is that he slipped and fell, may have hurt his knee, and was being treated by the side of the pitch.
"He's been taken to hospital."
After the game Chelsea cordoned off the section of the stand where the fans vented their anger at the United players.