The Football Association is looking into remarks made by Manchester United boss Sir Alex Ferguson about referees' chief Keith Hackett.
Ferguson claimed Hackett told referee Mark Halsey to rescind the red card given to Chelsea captain John Terry against Manchester City.
The United manager said Hackett "would never have done this for us".
Terry was exonerated on Tuesday and the referees' body said: "All matters of discipline are an issue for the FA."
Terry had been facing a three-match ban after Halsey deemed the tackle on City striker Jo was "serious foul play".
But the FA's Independent Regulatory Commission upheld Chelsea's appeal, meaning the England skipper is free to face United in Sunday's Premier League clash.
That decision clearly riled Ferguson, who will be without Nemanja Vidic for the same game after the Serbian defender was sent off against Liverpool on Saturday.
For me it is a surprise, a positive surprise
Chelsea boss Luiz Felipe Scolari on the Terry decision
"My information is that Hackett told Halsey to rescind the red card and he would not do it," said Ferguson.
"If it had been a Manchester United player Hackett would never have done this for us."
Halsey has been removed from the list of Premier League referees for this weekend and will instead take charge of the League Two encounter between Chester and Shrewsbury.
If the FA decides to investigate Ferguson's remarks further, it will write to the United manager asking him to explain them.
BBC Radio 5 Live sports news correspondent Gordon Farquhar explained: "There is a fairly high threshold for what constitutes opinion and debate about football and for an offence to be triggered it would have to be reasonably serious.
"The FA would see an accusation of bias like this as being something which required further investigation.
"The process would now be for the FA to write to the person who made the remark and ask for their clarification and any further disciplinary action would follow on from that - but that is some way off at this point."
Hackett is yet to comment, but a spokesman for referees' body the Professional Game Match Officials Board, of which Hackett is general manager, said he had nothing to do with the decision to rescind Terry's red card.
It is not the first time Ferguson has directed his wrath at Hackett.
After United's FA Cup exit at the hands of Portsmouth last season, Ferguson launched a stinging attack on referee Martin Atkinson's handling of the game and said Hackett was "not doing his job properly".
On that occasion, the FA found him not guilty of improper conduct.
Chelsea boss Luiz Felipe Scolari admitted he was pleasantly surprised that Terry had been cleared.
"I am satisfied because it is the first time I am a coach when they changed the decision of the referee," he said.
"I think only in England this would happen. In other countries the referee is the God.
"This time the referee said he made a mistake. For me it is a surprise, a positive surprise. The Football Association understands that the referee makes mistakes sometimes like you and me."
It was initially thought that Halsey had dismissed Terry for a professional foul on Jo, but he later clarified that the offence was "serious foul play".
That is described by world governing body Fifa as using "excessive force or brutality against an opponent".
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