Manchester United defender Jonny Evans has denied spitting at Papiss Cisse, while the Newcastle forward has apologised for his actions.
Evans and Cisse were involved in an incident five minutes before half-time in Wednesday's Premier League fixture between their sides at St James' Park.
Television pictures appeared to show Evans spitting at Cisse as the Senegal international rose from the turf after the pair tussled for the ball with the game goalless.
Cisse stretched out to grab his rival in response and seemed to spit back in Evans' direction.
The FA is already probing the incident and will study the referee's report and review the match footage before deciding whether to take any action against the pair.
Evans has strongly denied any wrongdoing in a statement issued on United's official website on Thursday afternoon shortly before Newcastle published an apologetic statement from Cisse.
"Having woken up this morning I am shocked to have seen the media coverage from last night's match. I would like to make it clear that I did not spit at Papiss Cisse," Evans said.
"It is not in my character or in my nature to spit at anybody nor is it something I have ever done or would ever do."
"I was totally unaware of any spitting incident and had assumed that the issue at the time was with the challenge and his attempted retaliation to the tackle from the floor.
“During the game Papiss Cisse and I spoke about the incident and it is clear by my reaction in the television footage that I was totally surprised by any suggestion of spitting.
"It is not in my character or in my nature to spit at anybody nor is it something I have ever done or would ever do. It is certainly not something that I did last night."
Cisse said in his statement: "I have apologies to make to a lot of people today. Firstly to my team-mates and to our supporters, secondly to Jonny Evans, and thirdly to every football fan who saw the incident between myself and Jonny.
"I reacted to something I found very unpleasant. Sometimes it is hard not to react, particularly in the heat of the moment. I have always tried hard to be positive a role model, especially for our young fans, and yesterday I let you down.
"I have always tried hard to be positive a role model, especially for our young fans, and yesterday I let you down."
"I hope children out there playing football for their clubs and schools this weekend will know better than to retaliate when they are angry. Perhaps when they see the problem it now causes me and my team they will be able to learn from my mistake, not copy it."
If the FA decides to charge the pair, their bans will be lengthy if they are found guilty following a change to the rules at the beginning of the season.
Both could be facing a six-match suspension, after the FA fell into line with FIFA which demands that term for such an offence.
However, Cisse could be forced to sit out seven matches having already been suspended by the FA for violent conduct earlier in the season.