Wales in World Cup breach
Teams are only allowed to utilise the 31 players in their declared squads.
Injury-hit Wales are believed to have drafted in the extra players at a training session on Wednesday.
The Welsh play hosts England at Twickenham on Saturday in a crucial Pool A clash.
"The Wales team has been issued with an official warning following an apparent breach of the spirit of the Rugby World Cup 2015 Terms of Participation regarding team training arrangements and specifically the inclusion of players from outside the selected 31 player squad," said a statement.
"Having reviewed the matter, World Rugby is satisfied that no deliberate breach was intended," added World Rugby.
World Rugby said all 20 nations at the World Cup had been reminded about the rules.
Coach Warren Gatland insisted that Wales have done nothing "underhand."
"Apparently from the statement from World Rugby, we have broken the spirit of the rules," Gatland said.
"What that means I don't know. Look, we didn't do anything that we thought was out of hand.
"We brought some players in for training for a little bit of defence work, so I don't know what we've done wrong," Gatland added.
"We didn't try and do anything underhand. If we were going to try and break the rules, we would have been sneaking players in."
Agence France-Presse