England flanker Tom Curry has said he was racially abused by a South Africa player during their Rugby World Cup semi-final in Paris on Saturday night.
While the alleged slur is not audible on the referee's audio link, a subsequent conversation Curry had with referee Ben O'Keeffe is.
The incident is said to have involved South Africa hooker Mbongeni Mbonambi.
Curry was heard saying to the referee, at about the half-hour mark: "Sir, if their hooker calls me a white c***, what do I do?"
O'Keeffe replied: "Nothing, please. I'll be on it."
Mbonambi refused to shake Curry's hand at the end of the match, which South Africa won 16-15.
Asked after the match if Mbonambi had said something he shouldn't have, Curry replied "yeah", although he declined to reveal what it was.
World Rugby has confirmed there is a 36-hour window from when the game ends for England to lodge a complaint to the citing officer.
It means the deadline is 10am on Monday.
The law that applies in such alleged cases is 9.12 - that a player must not verbally abuse anyone.
The sport's global governing body can also choose to pursue the matter itself - in which case it is likely to be dealt with under the banner of "bringing the game into disrepute".
South Africa forwards coach Deon Davids was asked about the incident at his side's media briefing on Sunday morning.
"I'm not aware of that," he said.
"I'm not aware of any comment. If it was discussed, I'm not sure what the comment was or when it was said. I don't know."