Chelsea have lodged an official complaint over alleged abusive language used by referee Mark Clattenburg towards two of their players during their 3-2 defeat to Manchester United.
The official had a controversial game, sending off two Chelsea players and allowing Javier Hernandez's winning goal to stand, despite the striker being in an offside position.
Chelsea have not revealed the details about the incidents involving Clattenburg but have released a club statement confirming that they have lodged an official protest with the Premier League.
The statement read: "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 on to the Football Association. We will make no further comment at this time."
One of the two players is understood to be John Obi Mikel. The nature and content of the comments are as yet unclear.
The Professional Game Match Officials (PGMO) has stated that it is treating the complaint "with the utmost seriousness". Clattenburg, meanwhile, has said he'll co-operate fully and "welcomes the opportunity for the facts to be established".
Chelsea manager Roberto Di Matteo was late out to his post-match press conference and it is understood he was discussing the match's several controversial incidents with Clattenburg.
The Daily Mirror claims Mikel was seen marching into the referee's room after the game to launch a rant at the official. Chelsea captain Terry, Di Matteo and the club's chief executive, Ron Gourlay, were also reported to be in the room shortly before the club confirmed they would make their formal complaint.
Meanwhile, Chelsea have launched an investigation into reports that a steward was injured by objects that were thrown onto the pitch following Manchester United's winning goal.
Chelsea fans in the Matthew Harding stand, behind the goal in which Hernandez, 24, scored, hurled abuse and appeared to throw objects at the United players as they celebrated.
The steward, who was later taken to hospital, fell to the floor and needed approximately 10 minutes' treatment by paramedics at the side of the pitch.
"We are looking at reports of objects that may have been thrown or incidents around the steward taking a fall," a Chelsea spokesman said. "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."
Hernandez appeared to gesture towards the set of home supporters after his goal, with Patrice Evra trying to calm him down immediately afterwards.
More stewards came to the aid of their injured colleague, before a team of paramedics also came to help him. After the game Chelsea cordoned off the section of the stand where fans had vented their anger at the United players.
Information from the Press Association was used in this report