The Ireland centre was sent off in Ireland's final pool game against Samoa for a high tackle, and will now miss his side's quarter-final against New Zealand.
Aki contested the decision and attempted to have it overturned, but a disciplinary panel upheld referee Nic Berry's red card decision, and gave Aki a six-match ban.
That ban is reduced to three games due to the centre's clean disciplinary history and good conduct and behaviour.
"In our opinion the referee was correct to issue the Red Card because, consistent with World Rugby’s High Tackle Sanction Framework, the Player executed an illegal high tackle on his opponent, the ball carrier, because his left shoulder came into clear contact with his opponent’s head," World Rugby's ruling said.
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"The degree of danger to the Player’s opponent was high as there was forceful contact to his head requiring a head injury assessment and removal from the Match.
"In our opinion, there were no clear and obvious mitigating factors to warrant a sanction less than a Red Card."
Auckland-born Aki will therefore miss Ireland's quarter-final against the country of his birth, and the semi-final and final should Joe Schmidt's side progress.
If not, Aki will miss his provincial side Connacht's first two games after the centre's return.
The player and Ireland have 48 hours to appeal the ruling.
Should that appeal fail, Aki's suspension means Robbie Henshaw and Garry Ringrose will likely combine in Ireland's midfield against the All Blacks, with Chris Farrell the other centre option in the squad. Ireland cannot replace Aki with another player in their 31-man squad.
An IRFU statement said: "The Ireland Management are disappointed with the outcome of Bundee’s hearing and will review the Judicial Committee’s written report once received."
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