Fans of Manchester City are being urged to stop using "appalling" chants about the Munich air disaster.
Twenty-three people died in the Munich air crash
The official supporters club and other groups have written an open letter to fans urging them to support their team rather than goad Manchester United.
Twenty-three people, including eight members of the Busby Babes, died in the crash near Kirchtrudering in 1958.
The letter, titled "We are City, let's be proud to be blue", called those who use the chants "an embarrassment".
Authors include the Centenary Supporters Association, Mcivta newsletter and the official Manchester City Supporters Club.
Chairman Alan Garry was a school boy at the time of the crash, which also claimed the life of journalist and former City goalkeeper, Frank Swift.
"I think it's up to somebody like myself to educate people about what a tragedy and a terrible day it was," he said.
"We (the official supporters club) are taking a lead in trying to stop these people doing it. A lot of us cringe when we hear it and we are trying to do anything we can to stop it.
"It has gone on a long, long time and it's about time in our view that it stops.
"We can only try. It is nasty, and its repugnant. We only hope they'll listen and learn and stop doing it."
The open letter appeared in the programme for Manchester City's match against Chelsea and will be republished for the next game against Tottenham.
The next Manchester derby is at the City of Manchester stadium on 14 January.