The handling of an incident involving Wales' Tomas Francis in Saturday's clash with England has been criticised as a "clear and flagrant breach" of head injury assessment protocol.
Prop Francis sustained a head injury in the 20th minute of the Six Nations contest at Twickenham.
In an open letter to World Rugby, the Welsh Rugby Union and Six Nations Rugby, Progressive Rugby, a lobby group on player protection, said: "Francis attempts to stand and falls back and struggled to get to his feet.
“He is seen holding his head. He is then seen using the post for support. He displays clear signs of ataxia and of being dazed. He is seen by a physio/medic – it is not clear which.
“Francis displayed clear symptoms/indications under the World Rugby Head Injury Assessment protocol that necessitate the immediate and permanent removal from play. No HIA was needed.
"While, after a delay, Francis was removed from the field he was, incorrectly, allowed to undergo an HIA assessment and returned to the field, playing until the 56th minute, at which time he was permanently replaced by Leon Brown.
“Progressive Rugby is concerned that the events demonstrate that individuals who have incurred brain injury have continued to be permitted to play which will place them at likelihood of risk.
“We consider the above incident demonstrates a clear and flagrant breach of HIA protocol that potentially puts both the short-term and long-term health of an elite athlete at risk.”
The letter added: "In addition, we still await the outcome of investigations into the failure to remove [England's] Jack Nowell from the field during the game against Italy (Feb 13) and behaviour of the England medical team at the culmination of the Italy v England U20 game (Feb 11).
“It is our major concern that the public has observed that in Rugby Union individuals have incurred brain injury and been permitted to continue to play.
“Until satisfactory explanations are provided, we remain unable to accept World Rugby’s assertion that player welfare is the game’s number one priority.”
It is understood that the Francis incident was already being examined by Six Nations Rugby before Progressive Rugby’s letter was published, while reviews into the other incidents are also ongoing.
A statement from World Rugby on Sunday read: “Player welfare is paramount for everyone involved in rugby and a collaborative approach is at the heart of ensuring all processes, such as the Head Injury Assessment (HIA) operational in elite competitions, are followed for the good of the game.
“As with other elite international competitions, World Rugby works alongside Six Nations Rugby, who conduct reviews following every Championship game, to identify and address any potential issues that arise from these matches.
“This approach is supported by a dedicated HIA review process, which is available to tournament organisers where required.
“Meanwhile, World Rugby can confirm that all match officials involved in the current Six Nations participated in an education workshop delivered by the Chief Medical Officer last week to reaffirm their critical role in supporting the HIA process.”