The club revealed "a number of adult supporters attempted to enter the stadium using a concession season ticket" and the culprits, who were caught by stewards, had their tickets confiscated and have consequently been banned from Celtic Park.
A Celtic spokesperson told www.celticfc.net: "The club won't tolerate this and we will be unrelenting in our bid to stop this practice.
"These people are cheating fellow Celtic supporters and the club."
Prior to the game against Manchester City, the club warned that turnstile checks would be carried out as well as spot-checks on seats occupied by those in possession of a concession ticket.
Celtic warned they would continue to be vigilant with "the minimum of inconvenience to genuine supporters, who will appreciate why the club is taking such action".