The Government has urged Manchester football fans not to ruin a minute's silence to remember those who died in the Munich air disaster 50 years ago.
A clock at Old Trafford shows the time of the crash on 6 Feb 1958
The silence will be observed before United and City's Premier League match at Old Trafford on 10 February.
Sports Minister Gerry Sutcliffe said he "hoped the minute's silence will be dealt with in an appropriate way".
Despite fears some fans may not remain quiet, applause has been ruled out as an alternative tribute.
Responding to Sutcliffe's plea, MP for Manchester Withington, Liberal Democrat John Leech, said he did have some worries about the event.
"I do have concerns that a small minority of idiots will choose to ruin the minute's silence and I do think there is a case for having a minute's applause rather than a minute's silence to ensure that doesn't happen.
"Unfortunately, every football club has a few idiots who are prepared to ruin things for the vast majority of people.
"I would implore all City fans to ensure the minute's silence is kept perfectly."
Manchester City's official supporters club even took the step of writing to United to request applause rather than silence.
But United have said applause is inappropriate, given that 23 people died in the plane crash, in which seven United players were killed.
An eighth United player - Duncan Edwards - died from his injuries 15 days later.
And two other players - Johnny Berry and Jackie Blanchflower - never played again due to their injuries.