"World Rugby takes allegations of discriminatory behaviour extremely seriously," they said.
'We can confirm that we are formally reviewing the allegation made by England's Tom Curry in relation to the use of discriminatory language during the England versus South Africa Rugby World Cup 2023 semi-final on Saturday.
'World Rugby will not be making any further comment until the conclusion of the process.'
But though Mbonambi remains in the spotlight, the South Africa star is free to play in Saturday's blockbuster showdown against New Zealand.
No decision on a potential punishment is expected until after the tournament has finished, with the hooker denying any wrongdoing.
Since reporting the alleged slur, Curry has faced death threats online - as well as abuse.
His club, Sale Sharks, wrote on X: "Sale Sharks' owners, players, coaches and staff are absolutely appalled by the nature and level of disgusting abuse directed at Tom Curry and his family in relation to an incident in the England v South Africa World Cup semi-final.
"Everyone at the club stands fully and proudly behind Tom and we look forward to welcoming him back to Carrington."
Some rugby fans have suggested Mbonambi's comments could have been lost in translation.
It has been mooted that he may have been saying 'wit kant' - which translates to 'white side' - in reference to England and their shirts.
Curry refused to shake the hand of his opponent after England missed out on a place in the final.
He is now focused on helping Steve Borthwick's side finish third, with the Red and Whites set to play Argentina in the third-placed play-off on Friday.
And England attack coach Richard Wigglesworth has claimed the squad are supporting Curry.
"Tom [Curry] is good," he said.
"I know players will have got around him. Steve and the management team have been all over it.
"I'm sure he's getting every bit of support that he needs.
"I'm not sure you can ever put yourself in those shoes and know that feeling. What I do know is he's an impressive young man who, if selected on Friday, will pour everything into that England shirt as he has done on the previous 49 occasions.
"It will be dealt with by World Rugby. We get stuck into our prep for Argentina.