The Football Association have contacted Manchester United manager Sir Alex Ferguson and assistant Carlos Queiroz to ask them to explain their comments following the FA Cup quarter-final defeat to Portsmouth.

An irate Ferguson claimed referee Martin Atkinson's display was "unacceptable" and also accused referees' chief Keith Hackett of "not doing his job properly" in the wake of Saturday's 1-0 defeat at Old Trafford.

Ferguson and Queiroz, who also criticised Atkinson, were unhappy United were not given a first-half penalty for a challenge on Cristiano Ronaldo.

Both parties have until Wednesday, March 19 to respond.

United missed a succession of opportunities against Pompey at the weekend before Sulley Muntari netted a penalty after Tomasz Kuszczak fouled Milan Baros.

Kuszczak, who had earlier replaced the injured Edwin van der Sar, was sent off, leaving Rio Ferdinand to face the spot-kick.

But it was Atkinson's decision not to award a penalty when Sylvain Distin appeared to barge Ronaldo off the ball that provoked Ferguson's ire.

The United boss, who admitted a charge of using abusive language to referee Mark Clattenburg following the 1-0 defeat at Bolton earlier this season, also felt Atkinson let Portsmouth players get away with too many fouls and accused the referee of being "on their side".

Ferguson said: "It's absolutely ridiculous. I cannot explain it. Managers get sacked because of things like that and he's going to referee a game next week."

He then turned on Hackett, saying: "He's not doing his job properly and he needs to be assessed. I'm assessed as a manager, players are assessed, referees should be assessed properly by the right people.

"That performance today should not be accepted by our game."

Queiroz called Atkinson "a disgrace", while Ronaldo claimed after the game he is "scared" in games because he feels officials do not protect him.

But Harry Redknapp, the Portsmouth manager, today refuted any suggestion of refereeing bias in his side's Old Trafford victory.

He said: "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 don't know why he said what he did, you would have to ask him about that.

"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."