Chelsea have made a formal complaint about the language - reportedly racial - allegedly used by referee Mark Clattenburg and directed at two of their players during the controversial 3-2 home defeat to Manchester United.
The dramatic development, which came after a stormy top-of-the-table clash at Stamford Bridge on Sunday, will add fire to the two sides' League Cup match in London tomorrow night.
"We have lodged a complaint to the Premier League match delegate with regard to inappropriate language used by the referee and directed at two of our players in two separate incidents during today's [Sunday's] match," Chelsea said.
"The match delegate will pass the complaint to the football association. We will make no further comment at this time."
Sky Sports News claimed on Sunday that one of the players involved was Chelsea's Nigerian star, John Obi Mikel.
It was reported by the Daily Telegraph yesterday that Mikel accused Clattenburg of directing a comment of a racial nature at him.
Professional Game Match Officials, the body that represents Premier League referees, released a statement saying it was "aware of the allegations", adding they were "being treated with the utmost seriousness".
"Mark will cooperate fully and welcomes the opportunity for the facts to be established," it said.
All four officials - the referee, his two assistants and the fourth official - wore microphones and earpieces that allowed them to hear what each other was saying throughout the match.
But under Fifa guidelines, what is said between them is not recorded.
The complaint came after a highly-charged encounter in which Clattenburg dismissed two Chelsea players before Javier Hernandez secured the points for United with a goal that Chelsea manager Roberto di Matteo insisted should have been ruled for offside.
Goals from Spaniard Juan Mata and Brazilian Ramires had earlier hauled Chelsea back in the game after an own goal by defender David Luiz and a Robin van Persie strike had put them 2-0 down early on.
Branislav Ivanovic and Fernando Torres were sent off but it was the decision of Clattenburg to issue Torres a second yellow card for diving after a challenge by Jonny Evans that was the focus of much of Di Matteo's frustration.
The Spanish international had been booked for an earlier challenge on Tom Cleverley before his 68th minute clash with Evans.
"He played the ball between his legs, then he gets kicked on his shin and goes down - it's as simple as that," said Di Matteo.
"It's a free kick for us and might even be a sending off for Evans as he is through to the goal. I was shocked. I thought it was going to be a free kick for us and a foul for us, not against us.
"And the decisive goal was an offside goal. It's a shame that a game like this has to be decided in that manner by officials."
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