Relegated Dundee United have been deducted three points and fined £30,000 for using two ineligible players in their win over Inverness Caley Thistle.
Ali Coote, a 17-year-old midfielder, came on as a substitute, while Jamie Robson, 18, was an unused substitute.
SPFL rules forbid players from playing for their parent clubs if they return from lower-league loans after 31 March.
United chairman Stephen Thompson has resigned from his position on the SPFL board in protest at the sanction.
Having returned from loan spells from East Fife and Brechin respectively, Coote and Robson were not eligible to appear for United again until the start of next season.
'Punishment wholly disproportionate' - United
Dundee United pleaded guilty to breaches of SPFL Player Regulations 12, 14(i) and 14(ii), having failed to apply to register players as required by SPFL Player Regulations 7 and 3A.
But chairman Thompson has stepped down from his SPFL board role with immediate effect.
"Whilst acknowledging the club's error and accepting a breach of rules occurred, the chairman feels the gravity of the punishment is wholly disproportionate for what was a simple honest, human error," said a club statement.
"We await the full written judgment after which we will consider our next action.
"Our travelling fans are the ones who have been punished the most here and the club will examine ways to make this up to them."
Following United's relegation from the Premiership, Thompson would have had to seek re-election to the board for next season anyway.
The disciplinary hearing concluded that replaying the game, won 3-2 by United, before the end of the season - with the final round of games scheduled for this weekend - was not practical.
A statement from the SPFL also stated that the £30,000 fine was part suspended, without providing any more detail.
United are already seven points adrift of 11th-placed Kilmarnock, having been relegated on 2 May, while Inverness are still trying to finish seventh.
Coote, who made his United debut at the end of last season, played six games for League Two East Fife between 23 January and 23 April.