Manchester United are at loggerheads with Manchester City as hundreds of away fans look set to being locked out of next month's derby.
United are disgruntled at their rivals for failing to offer the full allocation of 3,000 at one end of the Etihad Stadium on April 7.
Sportsmail understands United officials have complained in writing to City to express displeasure as the row escalates.
City sources stress that they have been advised to limit the away end to 2,800 - just as their safety advisory group recommended for the Champions League tie against Feyenoord in November.