Chelsea have made a formal complaint against referee Mark Clattenburg after accusing him of using "inappropriate language" to two of their players in the controversial defeat to Manchester United.
The European champions confirmed they had reported Clattenburg to the Premier League match delegate after the fiery 3-2 loss at Stamford Bridge.
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 tonight insisted he would co-operate with any investigation.
Professional Game Match Officials (PGMO), the body which represents Premier League referees, tonight released a statement which read: "PGMO is aware of the allegations and they are being treated with the utmost seriousness.
"Mark will co-operate 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 15 minutes from time.