Hearts have admitted to fielding an ineligible player in their 2-1 Betfred Cup win at Cove Rangers on Wednesday.
Ann Budge, chief executive of the Jambos, has revealed Andrew Irving, who came off the bench in the 65th minute, was unregistered due to an administrative error.
The club’s fate will be determined at a hearing with the SPFL panel on Monday.
Clubs have previously been eliminated from the Scottish Cup for falling foul of the same rule but with the tie being Hearts’ opening match of a round-robin group format, the outcome of Monday’s hearing is unclear.
In a statement released on the club website, Budge said: “It is with much embarrassment that we have to advise that during last night’s Betfred Cup group stage match away to Cove Rangers, Hearts inadvertently fielded an ineligible player.
“Due to an administrative error on the club’s part at the end of the January transfer window, Andrew Irving entered the field of play in the 65th minute as an unregistered player.
“Andrew was given an extension contract in January, 2018 and his extension paperwork was all properly completed and in order.
“However, it was not loaded onto the online SFA registration system at the time. His official registration, therefore, ran out on June 9, 2018.
“Unfortunately, this was not picked up in advance of last night’s game.
“While stringent processes are already in place to try to prevent something of this nature occurring, clearly mistakes can happen.
“An additional step in our internal procedures will be implemented immediately to try to ensure this cannot happen again.
“The club would like to apologise to its supporters, to Cove Rangers Football Club and to the footballing authorities for this inadvertent breach of the rules.”