Forest Green have been deducted three points and fined £500 for fielding an ineligible player in their win over Southport.
Rovers beat the Sandgrounders 1-0 on Saturday, 9 August - the opening day of the Conference season.
At a hearing in Birmingham, they accepted the charge and offered to replay the game, a request that was rejected by the Conference panel.
The decision is subject to their right of appeal to the Football Association.
And, in a statement on their club website, Forest Green said they are "most likely" to appeal.
The deduction will see the club move down from ninth to 13th in the Conference table.