World Rugby is seeking clarification from Wales after a group of players from outside their World Cup squad reportedly took part in training on Wednesday.
It is understood that back-up players who do not make the final 31-man World Cup squad are not allowed to participate in training sessions with the official group.
No formal investigation has been launched by World Rugby, it's understood, but it is aware of reports and will speak with Wales.
Even if Wales were found to have breached any tournament rules during the training session in Surrey, there is no prospect of any points deduction being imposed, with a fine or warning the most likely outcome.
There is no suggestion that Wales deliberately intended to flout World Cup regulations.
In its statement, World Rugby said: "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 a number of players from outside the selected 31 player squad for a training session yesterday.
"Having reviewed the matter, World Rugby is satisfied that no deliberate breach was intended.
Wales continued their preparations at London Irish's new training facility in Sunbury ahead of tomorrow's crunch Pool A clash against England at Twickenham.
Wales head coach Warren Gatland (right) made a fierce defence in response to World Rugby's statement.
Gatland told reporters: "I am sure you've all read the statement.
"We haven't broken any rules. According to World Rugby, we've broken the spirit of the rules, so I don't know what that means. We haven't done anything underhand."