Alan Wiley has decided against suing Sir Alex Ferguson for defamation but expressed his "bitter disappointment" with the Manchester United manager's attack on his fitness.

Ferguson was given a four-match touchline ban, two of them suspended, and a £20,000 fine by the FA for saying Wiley "just wasn't fit enough" to officiate in United's 2-2 draw with Sunderland in October. The United manager has apologised for his remarks and referees' union Prospect confirmed Wiley would not be pursuing a case for defamation against Ferguson.

"Although I was bitterly disappointed about the personal attack on me I want to put this behind me and continue to concentrate on my refereeing," said Wiley.

Prospect has also written to the FA to seek greater support for referees and calling for stronger action if there are similar attacks in the future.

Prospect national secretary Alan Leighton said: "Unwarranted and untrue attacks on referees damage the hard-earned reputation of our members, call into question their integrity and damage the image of the game by suggesting that the referees are incapable of doing their job.

"Referees understand and accept that their decisions will be questioned, but personal attacks are unacceptable."

Leighton also defended Prospect against criticism that it had chose this case to become very publicly involved.

He added: "Prospect adopted a policy when we started representing referees four years ago that we would not make public comment on every criticism made by ex-referees or managers.

"The seriousness of the recent attack forced the union to depart from that policy. We will, however, continue to defend members where their professionalism is publicly questioned."