Fulham have complained to the Football Association after discovering that some Manchester United fans have been able to purchase seats for this weekend's FA Cup fourth-round tie equivalent to those on offer to Cottagers supporters £5 cheaper.
As is common practice for FA Cup games, the clubs had agreed a standard price of £45 for home and away fans but United members have been able to purchase them at a discounted rate of £40.
Fulham claim they were not informed of the discount and it is understood they would not have agreed to it had they known.
A United spokesman said: "Fulham fans are paying the same as United fans who are not members.
"A discount scheme has existed for members for around 20 years.
"We're surprised it has come as a surprise to them."
The FA has received the complaint and is understood to have written to United for a formal explanation.