Manchester United insist they took the "strictest possible measures" to prevent Rangers fans from buying tickets in home sections at Old Trafford.
United beat Rangers 2-1 in the Europa League on Thursday night and the Scottish club were supported by a sold-out away section, but their fans also acquired tickets in the home section.
Footage of Rangers fans chanting in a home section concourse went viral before kick-off and United fans reported that stewards warned them of their presence around the stadium.
It is prohibited for away fans to acquire tickets in home sections, and United's game against Galatasaray in the Champions League in October 2023 was marred by 2,000 away supporters surfacing in the home section, which forced the club's chief operating officer to issue an apology.
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