The Football Association have written to Manchester United manager Sir Alex Ferguson, asking him to explain the comments he made about referee Alan Wiley following Saturday's 2-2 draw with Sunderland at Old Trafford.
Ferguson launched an astonishing attack on Wiley after the game on ESPN, insisting the official was lacking the fitness to keep up with a Premier League game and hadn't added enough time on at the end of the match.
Ferguson said: "I was disappointed with the referee. He didn't add on any time for the goal. He played four minutes and two seconds. He was also walking up the pitch for the second goal needing a rest.
"He was not fit enough for a game of that standard. The pace of the game demanded a referee who was fit. He was not fit."
The FA's discipline and governance division have reviewed Ferguson's outburst and have asked him to explain his post-match comments with the prosect of a charge of improper conduct hanging over the head of the Scot.
While the FA have halted bosses speaking about officials before matches, managers are free to discuss the refs and their performances, as long as it is in the context of the game. They must not, however, imply bias, question their integrity or comment on anything of a personal nature.
Wiley was reported to be "very upset" by the comments and is particularly aggrieved that his physical condition has been called into question, having passed a stringent fitness test before the start of the season and (like all top refs) undergoing regular checks.
The Professional Game Match Officials Limited (PGMOL) guidelines state that all referees have an individually tailored fitness regime and weekly training programme, which includes weekly records of their fitness and the refs must have the ability to run 40 metres inside 6.2 seconds.
"The fitness of [our] Select Group match officials is continually assessed throughout the season to ensure it is of the highest possible standard," a spokesman told the Guardian.
Professionals union Prospect, who represent referees, have defended Wiley, describing Ferguson's comments as "totally unwarranted".
Alan Leighton, the national secretary of Prospect, told Sky Sports News: "The referees never want to be the centre of attention in games and it is disappointing that yet again they are put in that position.
"My main concern is about the unfounded and totally unwarranted remarks about Alan Wiley's fitness. 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.
"I understand that 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."
Blackburn boss Sam Allardyce is also facing a charge after he criticised Peter Walton for not giving a penalty for Thomas Vermaelen's apparent trip on David Dunn in Blackburn's 6-2 defeat at Arsenal.
A spokesman for the FA said: ''We are reviewing the comments in their full context.''