Television pictures appeared to show the Wales winger being knocked unconscious after landing on his head following a tackle in the Premiership match on Dec. 3.
The decision to allow the 24-year-old, who had previously spent six months out with concussion injuries, to resume playing was severely criticised amid widespread concerns about player safety and prompted an investigation.
A three-man Rugby Football Union panel concluded that while protocol was observed, North should not have been allowed back since the club's medical team accepted he may have lost consciousness.
"The Concussion Management Review Group CMRG's view is that there was sufficient evidence to conclude not only from the video evidence but also George North's history and risk stratification that he should not have returned to the field of play," the report said.
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"The CMRG considered the welfare of North was always at the centre of Northampton's actions, and does not consider that the medical team (or the club) failed to complete the HIA (Head Injury Assessment) protocol nor intentionally ignored the player's best interests."
The panel recommended that North follows a graduated return to play to optimise his recovery, casting doubt over his comeback which British media reported could be against Sale Sharks on Friday.
"Northampton Saints accepts the conclusion that George should not have been allowed to return to the field of play. The club continues to offer George its full support and looks forward to seeing him on the pitch again as soon as he is ready," Northampton said.
The panel also made new safety recommendations including the possible introduction of a pitchside video reviewer.
(Reporting by Neil Robinson, editing by Ed Osmond)
