Manchester United have complained to England's Football Association about their allocation of tickets for January's FA Cup third round tie at Manchester City.
Under FA rules, United are entitled to 15% of the capacity of the 47,805-seat Etihad Stadium, which is 7,100 tickets.
However, City have offered 5,500, just over 11%, on the grounds of safety.
United have special dispensation from the FA, due to Old Trafford's 76,000 capacity, to offer just over 11% to visiting fans, which is 8,500 tickets.
The Red Devils are demanding their full entitlement for the tie.