Manchester United are investigating how two supporters managed to stay overnight at Old Trafford before Saturday's match with Arsenal.
The pair, reported to be foreign students, were part of an official stadium tour on the eve of the match, but managed to cut away from the group and stay at the ground.
The fans were found during a routine security sweep at Old Trafford, being caught by one of the pre-match check measures.
Police decided not to arrest the duo and United are investigating how the supporters managed to evade detection.
A club spokesman said: “We realise people will go to any lengths to watch a game at Old Trafford but these people went too far.
“The fact that they and their belongings had been searched before entry means we are confident there was no risk to safety.”
United were the subject of a high-profile security breach in May, when the season finale with Bournemouth was postponed due to a bomb scare.
The stadium was evacuated after the discovery of what proved to be a fake bomb, left behind following a security training exercise, sparked a safety alert.
