Manchester United have warned anyone "visibly or audibly" supporting Manchester City in a home area of Old Trafford on Tuesday will be ejected.
The clubs jointly decided to cut away allocations for the two-legged Carabao Cup semi-final on security grounds.
There are fears City fans could buy tickets in home areas after United put tickets on general sale after failing to sell out.
It is expected around 70,000 fans will be at the game.
"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," said United, who have also emailed their season ticket holders with a reminder that "all tickets must be used by Manchester United supporters".
Tonight's crowd will be the highest for a United game in the Carabao Cup for a number of years, but is still expected to be around 6,000 short of capacity.
It will exceed the 65,798 they got for the semi-final against Hull in 2017 but will fall short of the 74,576 they got for the 2010 semi-final against City, which United won thanks to an injury-time Wayne Rooney goal.
The failure to sell-out will cost United more than £200,000 in lost gate receipts and would almost certainly have been avoided had it not been for the joint decision to cut normal away allocations for such fixtures.
Instead of the 10% allocation as stipulated in Football League regulations, City received 3,000 tickets for the game, with United getting around 2,800 for the return leg at Etihad Stadium on 29 January.
The clubs took the decision following a number of incidents that took place when the sides met last month, when United midfielder Fred appeared to be racially abused by a City supporter.
It is also understood at the previous semi-final, there were flashpoints that made the clubs wary of the consequences of a repeat.
United say increased police, stewarding and crowd surveillance will be in operation 'both inside and outside the ground'.