But they defended their conduct in a statement last night in which they confirmed they were pursuing a complaint on behalf of Mikel but had decided not to proceed over allegations involving Mata.

The statement read: “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.

“The correct protocol for doing so was to report the incidents to the match delegate, and the club took immediate steps to inform him as soon as he was available. It was not a decision the club took lightly.

“Since reporting the matter to the delegate, the club and outside legal counsel have conducted further investigations.

“Having completed that process, we have now followed the correct protocol and lodged a formal complaint with the FA regarding John Mikel Obi.

“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.”

Press Association Sport understands Chelsea submitted signed witness statements from a number of players and staff to the FA, who had already begun their own probe into the matter.

The police are also investigating the allegations against Clattenburg, who it is understood categorically denies any wrongdoing.

Chelsea’s statement added: “We will cooperate fully with the FA and the police as they each investigate the matter.

“As events on Sunday are now subject to those investigations, we are unable to comment further until those processes are complete.”

The FA did not respond last night to Chelsea’s announcement as they continued their probe, while the Metropolitan Police were not immediately available for comment.

It is thought Clattenburg had yet to be interviewed this morning by either organisation but he and his fellow referees are said to have been shocked and angered by the claims against him.

The 37-year-old had already vowed to assist with any inquiry.

Chelsea refused to comment on reports neither Mikel nor Mata had heard Clattenburg abuse them but were rather told of the allegations by team-mates.

Sources have told Press Association Sport Clattenburg, his assistants and fourth official were stunned after being confronted by an angry Mikel in the referees’ room after Sunday’s match.

Clattenburg’s colleagues, who were miked up to him throughout the game, were all said to have denied hearing him abuse either player.