World Rugby will seek further information from England‘s Rugby Football Union about an "apparent non-compliance" with its protocols over the handling of George North's recent head injury.
A joint RFU and Premiership Rugby investigation opted against charging Northampton or their medical staff with misconduct for allowing North to resume playing, despite the Saints winger appearing to have been knocked unconscious in a league match on December 3.
In a statement released on Friday, World Rugby said it was "disappointing" that its protocols appeared not to have been followed, with North remaining on the pitch in the match against Leicester at Welford Road.
Replays appeared to show that North had been knocked out following an aerial collision with Leicester's Adam Thompstone, yet he passed a head injury assessment (HIA) and resumed playing.
Saints' defence was that not all replay angles of the incident were available at the time of assessment and that the Wales wing denied any loss of consciousness, had immediate recall of events and stayed motionless due to concerns for his neck pain.
The Concussion Management Review Group found that, while North should have been removed from play, medical staff were not wilfully negligent.
In a statement, World Rugby said: "Our number one priority is player welfare and our primary concern is the welfare of George and all players.
"It is therefore disappointing that apparent non-compliance with the sport's elite head injury protocols led to George remaining on the field of play when he should have been immediately and permanently removed.
"World Rugby's recognise and remove protocols state in no uncertain terms that any player displaying clear or suspected signs of loss of consciousness should be permanently and immediately removed from the field of play.
“In this case, the Head Injury Assessment should not have been undertaken.
"Unions and tournament organisers can access the Head Injury Assessment process for elite competitions, subject to World Rugby's player welfare tournament standards criteria being met.
"In this incident, it is clear that there was a failure to comply with these robust and well-established protocols, which continue to successfully protect players around the world.
"Specifically, failure in the identification and management of the injury and due consideration of risk when considering North's history were at fault in this unfortunate case."
The statement added: "While recognising that there is always a risk of human error, we will seek further information from the RFU regarding the factors behind the apparent non-compliance with World Rugby protocols and the outcomes reached by the independent review group."
In this case, the Head Injury Assessment should not have been undertaken
Northampton opted against selecting North for Friday's Aviva Premiership clash with Sale at Franklin's Gardens.
Director of rugby Jim Mallinder revealed earlier this week that the 24-year-old British and Irish Lions wing was in contention to start, but had been omitted as crisis club Saints seek the victory that will ease the pressure heading into Christmas.