The Football Association have written to Manchester United manager Sir Alex Ferguson to ask him to explain his comments about referee Alan Wiley.
Ferguson claimed the 49-year-old official was unfit after taking charge of his side's 2-2 draw with Sunderland at Old Trafford on Saturday.
Having looked at the remarks, the FA have now requested that the United boss explains the reasons behind his outburst.
However, no further action will be taken against Blackburn manager Sam Allardyce who criticised Peter Walton for not giving Rovers a penalty in Sunday's 6-2 defeat at Arsenal.
Ferguson's comments have been criticised by Alan Leighton, national secretary of professionals' union Prospect - who represent referees.
"My main concern is about the unfounded and totally unwarranted remarks about Alan Wiley's fitness," he said. "Not only does it dig at the heart of Alan's professionalism, where he is one of our top and longest-serving referees, but it also has an impact on all the others.
"Sir Alex made comments about other referees and said about European referees being fit as butchers' dogs, comparing them with some referees from the UK who aren't fit.
"All of the referees, all of the officials and all of the assistant referees pass stringent fitness tests at the start of the season.
"Prozone statistics are used to monitor their performance in every game. If Alan Wiley or any other referee were not fit they would not be refereeing."
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