Everton boss David Moyes was warned by the Football Association over his future conduct at a London hearing following remarks made after the Merseyside derby with Liverpool last October that ended with the Toffees reduced to nine men.
Tony Hibbert and Phil Neville were sent off and Everton also claimed they should have had a late penalty for a foul on Joleon Lescott by Jamie Carragher.
Moyes was heavily critical of referee Mark Clattenberg and also suggested Liverpool skipper Steven Gerrard was implicated in the red card for Hibbert, saying after the match: 'The referee pulled out a yellow card and then the Liverpool captain has a word with him and it becomes a red.'
Moyes made comments in the after-match media conference, and then again in the build-up to the next match against Larissa in the UEFA Cup the following week.
Moyes was also disappointed with an exchange he had with Clattenburg after the derby defeat, Liverpool winning with two penalties from Dirk Kuyt.
Now the FA's regulatory commission has considered Moyes' comments and issued their warning.
Moyes was found guilty of improper conduct in relation to comments made at a press conference in October last year, following the Merseyside derby, won 2-1 by Liverpool.
A statement from the commission said: 'Having considered all of the evidence and submissions, the commission concluded that the comments would reasonably be interpreted as calling into question the referee's integrity and implying that he was motivated by bias.
'The commission recognised the charged atmosphere of the game and the frustration that Mr Moyes was feeling.
'It also took into account his exemplary disciplinary record. However, the commission also stated that it is very important for managers to exercise great care in making comments about referees as they set an example for others.'