United's home leg will take place on Tuesday evening, and will be the first of two games which have seen both away allocations cut on security grounds.
However, due to poor sales tickets were released on general sale meaning anybody could buy them, including City fans wishing to attend.
In football league games, away fans are entitled to 10% of the total allocation for a match, but City have been given 3,000 for this tie, with United getting 2,800 for the return leg on 29 January at the Etihad.
United have made it clear that anybody found rooting for City in a United will be ejected, saying: "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.
"United, who have also emailed their season ticket holders with a reminder that "all tickets must be used by Manchester United supporters".
Fred and Kevin De Bruyne
Image credit: Getty Images
Around 70,000 people are expected to attend the semi-final first leg, which is 6,000 off Old Trafford's capacity.
By some calculations the failure to sell out the ground will cost the club around £200,000.
Both clubs claim that controversies over apparent racist abuse of United midfielder Fred in last month's league game, in addition to other incidents, led them to take the decision, and there will be an increased police presence as well as stewarding and crowd surveillance.
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