Manchester United have released a statement warning Manchester City fans that they will be ejected from Old Trafford if they are discovered in the home section during the Carabao Cup semi-final first leg on Tuesday night.
City's away allocation was cut to just 3,000 following fears that there could be trouble between the two sets of fans, but this has led to worries that some City supporters may buy seats in the areas designated for United fans.
There will be an increased police presence and crowd surveillance in operation as both clubs try to avoid any flashpoints.
United said: "We have a series of measures in place to minimise any risks and anyone visibly or audibly supporting the away team in home areas will be denied entry to the ground or ejected during the game."
They also emailed all season ticket holders to remind them that "all tickets must be used by Manchester United supporters".
The fixture has see a number of unpleasant incidents take place in recent times. Last month a City fan appeared to racially abuse United midfielder Fred during the Premier League game at the Etihad Stadium between the two teams, and the player was also hit by an object thrown from the crowd.
Supporters also clashed during the derby in 2016, with 11 arrests being made, while former United defender Rio Ferdinand suffered a head wound after being hit by a coin thrown from a City fan in 2012.
City are looking to win the Carabao Cup for the third time in a row, while United are seeking their first piece of silverware since they won the Europa League in 2017.
Jesse Lingard, Anthony Martial and Luke Shaw are fighting to be fit for United, while Nicolas Otamendi is available again for City after recovering from injury.
