Sir Alex Ferguson to offer second apology over Alan Wiley attack

• Manager's explanation of remarks due to FA by Friday
• Referee's union calls for Uefa-style match-day ban

Sir Alex Ferguson is to offer a second apology over his controversial comments about Alan Wiley's fitness, this time to the Football Association, in an attempt to avoid a misconduct charge.

Manchester United's manager has until Friday to explain to the FA the personal attack on Wiley that followed his team's draw with Sunderland on 3 October, when he publicly questioned the official's fitness for Premier League games.

Ferguson sought to defuse the subsequent furore at the weekend when he issued an apology on the club's website for "any personal embarrassment" his remarks may have caused Wiley and he is expected to offer a similar but more detailed apology – again stopping short of an unreserved one – in his submission to the FA. A letter is understood to have been drafted by United for his approval on return from a holiday in New York.

The FA will not be swayed by Ferguson's initial apology and will only decide whether to charge him once it has received his written explanation for the outburst. It is expected that the United manager will be charged, particularly as the governing body wrote to every Premier League manager at the start of the season to instruct them not to comment about match officials.

The initial apology was dismissed as "half-hearted" by the professionals' union, Prospect, today. The union has called for Ferguson to be severely sanctioned for his comments with a charge of defamation also under consideration. The FA has declined to comment on the demands or the Scot's apology, and insists its response will come only after its Disciplinary and Governance Unit has considered Ferguson's explanation.

"I think it's a half-hearted apology at best really, and it probably exacerbates the position, rather than resolving it," Alan Leighton, the national secretary of Prospect, said. "He clearly hasn't retracted the statement about Alan being unfit so it's not an apology for the main offence caused, and then he widens it to question the fitness of other referees, so he seems to be opening another can of worms which I don't think is very helpful at all.

"Referees are very fit. They have sports scientists who test them regularly throughout the season. They don't just pass a fitness test at the start of the season. Their body fats and BMI are regularly monitored, there are get-togethers every two or three weeks where they are put through extensive training and testing."

Ferguson is facing a possible fine or touchline ban for comments that reportedly led Wiley to consider his future in the game. Prospect, however, believes the United manager should be banned from having any contact with his players on match-days.

Leighton added: "I think the punishment should be a Uefa-type coaching ban, which is rather more than a touchline ban. Referees always accept decisions are going to be pored over. They have no problem with legitimate criticism. What's problematic is when the integrity and key components of refereeing are being questioned in a totally unwarranted and unfounded way, and we will defend our members when they are."

Ferguson is said to be furious at what he sees as both a media witch-hunt against him and a campaign by the refereeing fraternity to strike at him. Official ProZone statics showed that Wiley ran further than all but four United players in the match against Sunderland.

Wiley is set to take charge of Wigan's high-profile game at home to Manchester City on Sunday, in a sign of backing from the referees' manager, Keith Hackett.