The Ospreys have been fined £100,000 and docked four league points next season for breaching Magners League rules.
It relates to the postponement of their Magners League clash against Ulster at Ravenhill on March 19, when the Ospreys claimed they lacked sufficient front-row players to fulfil the fixture.
But a three-man Celtic Rugby disciplinary panel reached its decision following a complaint lodged by the Magners League tournament director.
“The panel found the Ospreys breached tournament rules, and consequently their actions amounted to misconduct,” said Celtic Rugby in a statement.
Ospreys, who host Glasgow in a play-off clash tonight, have the right of appeal.
“We will now await the full written judgement before making a decision whether to appeal against the verdict,” said an Ospreys spokesperson.
The Ospreys would have been without five front-row players for the Ulster game, originally postponed in January due to bad weather. Three were involved in Wales' RBS Six Nations game against Italy on March 20, and another two were injured.
The Ulster match eventually took place on April 13, Ospreys winning 38-27.