Houllier said: "It is not only the referees, but sometimes linesmen's decisions have been absolutely appalling. We conceded two goals at Bolton, one was offside and their second was handball.
"Then it happened at Chelsea when El-Hadji Diouf was sent off by a mistake from the linesman - it is a good job we achieved the victory we wanted because that red card was rescinded.
"The levels of some of the assistant referees is just not good enough. I am not blaming them, it is a difficult job. But when an offside is maybe half a yard and missed you can say, `okay, these things happen and we understand'.
"But when it's two or three yards, something is wrong somewhere. I know what I can do to improve my players; regarding referees, you must ask Keith Hackett at the FA."