The FA is considering launching an appeal after Sir Alex Ferguson and Carlos Queiroz were found not guilty of improper conduct after the FA Cup quarter-final defeat to Portsmouth.

Ferguson faced the charge after describing the performance of Martin Atkinson, the referee, during his side's 1-0 loss was "unacceptable". He also accused Keith Hackett, the head of the Professional Game Match Officials Board, of "not doing his job properly".

Ferguson and Queiroz, who also criticised Atkinson, were unhappy United were not given a first-half penalty for a challenge on Cristiano Ronaldo. Speaking after the match, Ferguson said: "I cannot explain it. Managers get sacked because of things like that and he's going to referee a game next week." "

However, at an independent regulatory commission hearing yesterday the charges were found 'not proved'. The FA have confirmed they have the option to appeal - and refuted suggestions a transcription error led to the commission finding the pair not guilty.

The governing body will await receipt of the full judgement and "written reasons" before considering further action. As for suggestions their case failed because their belief Queiroz described Atkinson as a 'robber' proved to be incorrect, the FA insist the charge did not centre solely on that alleged comment.

"The charges were brought on more than just the issue of the word 'robber'," an FA spokesman said. "At any disciplinary hearing the FA, as the prosecutor, argues its case, as does the defence. The fact that the commission found the charge was not proved does not mean that the FA made any errors in putting the case together.

"The commission has to make a decision based on the applicable standards of proof, which is balance of probability."