The Ospreys have been fined £100,000 and docked four points from next season's Magners League for postponing their 19 March game at Ulster.
A Celtic Rugby disciplinary panel met in Glasgow and found they "breached the tournament rules and consequently their actions amounted to misconduct."
The Ospreys asked to postpone the game insisting they did not have sufficient front-row cover to fulfil the fixture.
The Welsh region won the rearranged game 38-27 to help them into play-offs.
There were initial fears that the Ospreys, who host Glasgow Warriors in the first Magners League play-off at the Liberty Stadium on Friday, would suffer points deduction this season which could have hindered their play-off bid.
But the three-man Celtic Rugby disciplinary panel - chaired by professor Lorne Crerar of Scotland - punished the Ospreys financially and ensure the Liberty Stadium region will start next season on minus four points.
The complaint was brought by Magners League tournament director David Jordan on behalf of Celtic Rugby after the postponement of the Ravenhill round 11 fixture.
An Ospreys spokesperson: "We will now await the full written judgement before making a decision whether to appeal against the verdict.
"There will be no further comment on this issue from Ospreys Rugby."
Both the Ospreys and Celtic Rugby have the right of appeal.
The Ospreys, though, had earlier stated they had never agreed to the 19 March date for the Ulster match, which was originally called off on 8 January due to bad weather.
The Ospreys claim they did not want the 19 March date because they had already given the players and management leave between 12 and 22 March and had given Ulster three alternative dates which had been turned down.
The Ospreys insisted they did not have any front-row players of sufficient quality to honour the fixture and were not prepared to use semi-professional players from their feeder clubs - Aberavon, Neath and Swansea - in the Principality Premiership.
Front-rowers Adam Jones, Huw Bennett and Paul James were all in Wales' matchday squad for their final RBS Six Nations game against Italy in Cardiff on 20 March, while fellow forwards Duncan Jones and Richie Hibbard were injured.
A Magners League statement read at the time: "This rule was introduced for health and safety reasons and Celtic Rugby could not sanction a match in these circumstances.
"The reason for the postponement is that Ospreys are unable to supply suitably experienced front-row players in their match squad as is required under the Magners League rules and International Rugby Board rules."
It is not the first time the Ospreys have been investigated by rugby authorities this season.
European Rugby fined the Ospreys 25,000 Euros for having 16 players on the pitch against Leicester in a Heineken Cup match in January.
Full-back Lee Byrne, who was fined 5,000 Euros, came on the pitch illegally.
And then the Ospreys were in the dock again over scrum-half Ricky Januarie's contract but were told they had no case to answer.