Manchester United are reviewing their security protocols after a pitch invader entered Old Trafford with two toy guns during last month's Champions League game against Juventus.
Shortly after play got underway during the hosts' 1-0 defeat to the Italian champions, a fan ran across the pitch and was restrained by stewards close to the players.
The Daily Mail reported Old Trafford security staff later inspected his seat and found two toy guns in a backpack.
Manchester United said the safety of fans was an "absolute priority" for the club.
A spokesman added: "Carrying a children's miniature plastic toy gun into a stadium is clearly an extremely 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 into the circumstances around this issue. This includes a full review of our already-rigorous security protocols, using our own security teams together with external expert consultants."
At the time of the incident, Greater Manchester Police confirmed a man in his 40s was kept in custody overnight, having been arrested on suspicion of pitch invasion.
The pitch invasion was one of three on the night which has seen United charged by UEFA with the case to be dealt with by the UEFA Control, Ethics and Disciplinary Body on November 22.