Manchester United are being taken to court by a supporter over the club's automatic cup ticket scheme.
Man Utd have already sold most of their allocated season tickets
The policy forces season-ticket holders to buy tickets for all cup matches.
A fan, John Mayall, is unhappy that the club agreed to renew two season tickets but then said he would not get them unless he joined the ACS scheme.
Mayall, who is backed by the Manchester United Supporters Trust, has now issued papers to the County Court, which need a response from the club by 19 July.
Mayall is a MUST member says it is unfair to expect him to buy tickets for all home matches, at an unspecified cost, as he finds it difficult to attend games in midweek - when the majority of cup matches are played.
it is inherently unfair to force fans to buy tickets they do not want or cannot afford
Ironically, the ticket policy was initially introduced on a voluntary basis to help supporters who found it difficult to meet deadlines for ticket applications, which were often arranged at short notice.
But United's decision to extend the ACS to all remaining season-ticket holders has prompted criticism from certain fans.
"We don't recommend taking legal action against our club lightly but attempts to change minds about this have been brushed off," said MUST chairman Nick Towle.
"We call on United to return to the status quo now and retain the ACS as a voluntary scheme because it is inherently unfair to force fans to buy tickets they do not want or cannot afford."
United have declined to respond to the threat in public, although they have acknowledged receipt of Mayall's letter.