The capacity for the West Midlands derby between Birmingham City and Aston Villa on Sunday, 30 October has been limited to 25,000 on safety grounds.
The Championship match - the first second-tier meeting between the rivals since 1987 - is a 12:00 GMT kick-off.
The full capacity at St Andrew's is 30,016 but will be reduced by a sixth.
A statement on Blues' website said: "The decision has been agreed after consultation between West Midlands Police and the club's safety team."
The two sides have not faced each other at Birmingham's home ground since 2011, when both teams were in the Premier League.
When they met at St Andrew's in a League Cup quarter-final in December 2010 in front of 27,679, the violence that erupted after full-time was described as "like a warzone" and then-Birmingham boss Alex McLeish said the trouble threatened to take English football "back to the dark ages".
Blues are currently sixth in the table with 20 points from their first 11 games, while managerless Villa are 19th with 10 points fewer.