The 1-0 defeat to the Serie A champions was marred by invasions both during the match and after full-time, with the object of reaching Juventus’ former United star Cristiano Ronaldo the apparent target on each occasion.
And while the toy guns in question were not carried on to the field, they were later discovered in the backpack of one of the perpetrators.
A United spokesperson said: “Carrying a children’s miniature plastic toy gun into a stadium is clearly an irresponsible act. Fortunately, the actions of the pitch intruder at the Juventus game did not harm anyone.
“However, as the security and safety of all fans and staff is the absolute priority at Manchester United, the club immediately launched an investigation.
“This includes a full review of our already rigorous security protocols, using our own security teams together with external expert consultants.”
United were informed the day after the match that they were being charged under article 16 of the UEFA Disciplinary Regulations relating to field invasions, and the case is due to be dealt with by the governing body’s Control, Etchics and Disciplinary Body on November 22.
This is the latest in a string of security issues at the stadium, with the most high profile being the May 2016 incident which saw United’s home fixture against Bournemouth being postponed for two days when Old Trafford had to be evacuated following the discovery of an apparent explosive in a toilet.
It later materialised that the dummy device had not been returned following a training exercise earlier that week, with the external security company accepting full responsibility for the resulting postponement.Later that year a pair of United fans were able to wander off during a tour of the ground the day before a Premier League fixture against Arsenal and were able to evade security staff over night before eventually being caught moments into the match.