Rugby's world governing body has asked Six Nations organisers to explain the decision not to punish England prop Joe Marler for his "Gypsy boy" comment.
Marler made the comment to Wales prop Samson Lee, who is from the Traveller community, during England's Six Nations win at Twickenham on Saturday.
The Welsh Rugby Union said it was "surprised" Marler was not sanctioned.
Verbal abuse of a player based on ethnic origin can carry a minimum sanction of a four-week suspension.
In a statement Six Nations Rugby said it was "aware of the seriousness of Mr Marler's comment and does not in anyway condone what was said" but accepted it was said "in the heat of the moment".
World Rugby said in a statement that "in accordance with its regulations", it has requested further information from the Six Nations, "in respect of the process that led to its decision."
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Campaigners from the Traveller community - and figures inside the sport - had been critical of Marler for making the comment and called for a ban.
Harlequins player Marler, 25, has apologised to Lee, who said he accepted the comment as "banter".
Wales head coach Warren Gatland also described the incident as "banter", but subsequently apologised following criticism.
The WRU's questioning of the punishment caused England head coach Eddie Jones to accuse the WRU of trying to derail his side's Grand Slam bid.
Marler is on the bench for England's final game against France on Saturday as they bid to complete a Six Nations clean sweep, and World Rugby's intervention is not expected to affect the forward's ability to play this weekend.
World Rugby could start its own disciplinary procedures against Marler or ask the Six Nations to reassess its findings, should the organisation have concerns about the disciplinary process.