Rugby World Cup

Hosts: England Dates: 18 September-31 October

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Wales have received an official warning from World Rugby for a breach of Rugby World Cup regulations during training.

Back-up players who did not make the final 31-man squads are not allowed to take part in practice sessions.

But a group of players not in Wales' squad did participate during training at Sunbury on Wednesday.

"While World Rugby is satisfied that no deliberate breach was intended, an official warning has been issued to the WRU," the sport's governing body said.

"All participating teams have been informed that additional players from the extended squad, including potential injury replacements, may not be included in any training sessions."

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While there was little prospect of a points deduction for the infraction, with a fine likely to have been the sternest punishment, the incident will have been an unwelcome distraction as Warren Gatland prepares his side to face England on Saturday.

It was one of the first questions the coach had to field at Wales' Thursday afternoon press conference, with Gatland denying any wrong-doing.

"We haven't broken any rules," he said. "Apparently we have broken the 'spirit of the rules'. What that means we don't know... I don't know what we have done wrong."

The incident is also likely to have been embarrassing for the Welsh Rugby Union hierarchy, with World Rugby pointing out a breach of "the spirit of the Rugby World Cup 2015 terms of participation".

Nicky Smith, Rob Evans, Rhys Patchell, Jordan Williams, Dan Baker, Kristian Dacey, Dan Fish and Aled Summerhill were reportedly involved in the session at London Irish's Sunbury ground on Wednesday.

BBC Sport understands the non-squad players trained against the Wales team, which was named on Thursday, that will face England in Saturday's crucial Group A game at Twickenham.