The Twitter abuse of referee Mark Halsey has been condemned as by the match officials' union Prospect.
Halsey was the victim of two abusive tweets on Sunday, both of which referred to his throat cancer diagnosis in 2009.
The 51-year-old official sent off Liverpool midfielder Jonjo Shelvey and then awarded Manchester United a controversial penalty in the Premier League game at Anfield, with Robin van Persie converting the spot-kick for a 2-1 win.
The national secretary of Prospect, Alan Leighton, expressed his union's disgust.
He said: "Prospect wholeheartedly condemns the unacceptable abuse aimed at Mark Halsey on Twitter. The comments made by a very small minority will be seen as beneath contempt by all decent people.
"The main concern now is for Mark and his family, who are receiving the support of his employer, PGMOL (Professional Game Match Officials Limited). Prospect will also provide any assistance as required.''