Sir Alex Ferguson and his assistant Carlos Queiroz could face disciplinary action after the Football Association contacted the Manchester United duo demanding an explanation of their comments following last weekend's FA Cup quarter-final loss to Portsmouth.

In the aftermath of a 1-0 defeat that ended United's hopes of repeating the 1999 treble, Ferguson accused the referee, Martin Atkinson, of being on Portsmouth's side before launching into a tirade against Keith Hackett, the head of the Professional Game Match Officials Board. His sentiments were echoed by Queiroz, who branded Atkinson a "robber" and called for a new rule allowing for the dismissal of referees.

United's ire stemmed from Atkinson's failure to award a penalty against the Portsmouth defender Sylvain Distin, who escaped unpunished after barging Cristiano Ronaldo in the area. "I just cannot explain that decision," said Ferguson.

"Managers get sacked because of things like that and he [Atkinson] is going to referee next week? It's ridiculous. I don't blame Portsmouth. If you're going to come here you know you're going to get off with these things ... I think Keith Hackett's got a lot to answer for in this country. I don't think he's doing his job properly. He's got his favourites, as everyone knows."

"The referee is a disgrace," added Queiroz. "He deserves somebody to come to the side of the pitch after five minutes, give him a red card and pull him out of the game. That is my opinion of this robber. We did our job, Portsmouth did their job but the referee didn't."

But the Portsmouth manager, Harry Redknapp, today rejected any suggestion of refereeing bias during his side's victory at Old Trafford. "The referee was not on our side, that's for sure. He didn't have to be because there wasn't a bad tackle in the game. I don't remember one of my players making a bad tackle, not one, so I didn't agree with what Alex said there.

"He is entitled to his opinion after the game and if that is what he felt it is up to him, but I don't agree with it and that's the end of the matter. I only know what I saw and what I know, and as far as I was concerned I didn't see any of our players go in with their studs up or try to make a bad tackle all game."

Ferguson and Queiroz have until March 19 to respond to the FA, and will be fined if found guilty of questioning the impartiality of the officials.