Swansea City have warned fans they will be refused entry into the home end of their ground if they wear Manchester United shirts for Saturday's game.
The Premier League champions make their first visit to the Liberty Stadium, with the game a 20,520 sell-out.
The ban on United clothing was welcomed by the Swansea supporters' club.
The Swans, who have made a bright start to life in the top flight, are urging supporters to wear the club colours of black and white in the home end.
Swansea have told fans on their website: "Any supporters wearing Manchester United attire attempting to gain entry into the home sections will be refused entry, and will not be relocated to the away end due to Manchester United selling their full allocation."
Phil Sumbler, chairman of Swansea City supporters trust said the difference with seeing shirts from top flight clubs in the home section is that the Swans are now in the same division.
"I think it's been one of those generic observations that over the course of time it's not unusual to see Man Utd, Liverpool, Chelsea shirts at matches," he said.
"The difference is this season they are all the opposition.
"Swansea, like any city, has got a decent amount of Man Utd followers and you are not always going to get the chance to get to Old Trafford and they might see this as an opportunity.
"I'm sure anybody who is in the home end and wearing any Man Utd merchandise would be told by supporters before they are ejected."
Swansea City have enjoyed record sales of the club's shirt for their first season in the Premier League.
An initial allocation of 6,000 shirts sold out on the first day alone when they went on sale in the summer, with fans having to wait until September for the next batch to arrive.
After 30 years in the lower leagues since their last top flight game, Mr Sumbler said people were "once again proud" of supporting the Swans.
A Swansea City spokesman said: "It is club and stadium policy that we do not encourage away shirts in the home stands for any game, not just Manchester United.
"We cannot currently satisfy the ticket demands for Swansea City fans, who are our priority.
"Away clubs receive an allocation of around 2,000 tickets for the away section. Any away fans in the home section deny Swansea City supporters the opportunity to watch the game at the Liberty Stadium."