Manchester United will launch an investigation after a number of Rangers fans were able to watch Thursday's Europa League match from home sections at Old Trafford.
Over 3,500 Rangers supporters were officially in attendance in the away end for the game -- which ended 2-1 in the hosts' favour -- but television footage showed plenty more appeared to be in the home areas next to United fans.
Videos of away fans chanting in a home section also circulated on social media.
United said they put the "strictest possible measures" in place in anticipation of such circumstances but that "unauthorised touting of tickets cannot be completely eliminated."
The club, however, have promised an investigation with the "strongest possible sanctions taken" against registered ticket holders who may have sold theirs on.
"In anticipation of the risks around tonight's game, we took the strictest possible measures to prevent sales to away fans in home areas," a club spokesperson said.
"Tickets were only allocated to people who existed on our database prior to the draw taking place in August. Hospitality tickets had the same restrictions applied.
"Tickets forwarded by season ticket-holders and members also had the same restrictions applied [i.e. they could only be forwarded to fans on our database prior to the draw].
"Whilst the unauthorised touting of tickets has become more difficult since our introduction of in-app digital ticketing, it cannot be completely eliminated.
"All examples of away fans in home areas will be investigated over the coming days, with the strongest possible sanctions taken against the registered holders."
Following the match, Greater Manchester Police said 39 arrests were made across the night -- most in the city centre.
United won the Europa League tie following a late goal from Bruno Fernandes after Rangers had briefly cancelled out Jack Butland's own goal through Cyriel Dessers.