Manchester United will launch an investigation after a number of Rangers fans were able to get tickets in areas of Old Trafford designated for home supporters.
The club have also promised the 'strongest possible sanctions' for any registered ticket holders who might have helped sell them on.
Over 3,500 Rangers supporters were officially inside the stadium for the Europa League group stage match but significantly more appeared to have been able to secure a seat in the home end, resulting in complaints to United and the police.
Ahead of the game, United had put preventative measures in place to prevent away supporters from entering home sections.