Manchester United have complained to the Football Association about the number of tickets they have been granted for next month's FA Cup tie at Manchester City.
Under FA rules, United should be entitled to an allocation equivalent to 15% of the ground capacity at the Etihad Stadium, which would be 7,100 tickets.
Instead, City have offered 5,500 - exactly the same as United were granted for the first leg of the Carling Cup semi-final two years ago.
However, whilst that tie was agreed relatively quickly, the Red Devils are digging their heels in on this occasion and are demanding their full entitlement.
City are refusing on the grounds that it would constitute a safety risk, so the FA have been asked to intervene on the matter.
The issue came to light over discussions about the tie, and the potential for a replay.
United would not offer City 15% of Old Trafford's capacity for that.
However, they do have special dispensation from the FA due to the size of their stadium and City would be allocated 8,500 tickets - just over 11% of Old Trafford's 76,000 capacity.
The row will merely heighten the atmosphere around a powderkeg tie, which will see the pair - currently first and second in the Premier League - meet at City in the FA Cup for the first time since 1955.
After decades of playing second fiddle to their neighbours, City have enjoyed better fortunes of late, winning last season's FA Cup semi-final at Wembley and then hammering United 6-1 at Old Trafford this term.
Although that Wembley encounter passed off peacefully thanks to a massive police operation, there was trouble around the previous Carling Cup encounter at City, when 29 supporters were eventually arrested.