Rivals United and City are at war over who started the Manchester derby tunnel bust-up.
The FA have launched an inquiry into Sunday’s 20-man brawl that league leaders City claim was sparked by Man United boss Jose Mourinho demanding the visitors tone down their raucous celebrations following their 2-1 win at Old Trafford.
A milk carton was thrown at him in the melee, which broke out in the doorway of the away dressing-room after the United chief allegedly told the Blues’ players lower the volume of their loud music.
City coach Mikel Arteta suffered a cut eyebrow in the ensuing fracas in the narrow corridor outside the away dressing-room, with witnesses claiming United striker Romelu Lukaku was at the centre of the ugly episode.
The Reds deny the Belgian threw anything as tempers boiled over, but insist milk was thrown towards Mourinho from inside the City dressing-room, a charge denied by the visitors.
Both clubs have until 5pm on Wednesday to provide the FA with their written observations after it was confirmed referee Michael Oliver – whose room is adjacent to the away dressing-room at Old Trafford – did not witness the incident or include it in his report.
The two sides were intent on playing the blame game on Monday — City pointing the finger at Mourinho for sparking the incident, United claiming the over-the-top celebrations from the visitors were the catalyst for the post-match unrest.
One independent witness near the dressing room area said they had never heard music played so loud.
United are adamant Mourinho did nothing wrong and it has been suggested City captain Vincent Kompany was one of the main aggressors in the tunnel area.
The incident will serve only to increase the tension that already exists between the rival neighbouring clubs and old foes Mourinho and Guardiola.
It is understood there has been no dialogue between the two clubs since the incident, with no attempt from either side to draw a line under the episode that could see both hit with FA disciplinary charges.
It is the second time in a fortnight that City have irked opponents with their post-match celebrations following a victory.
Southampton took exception to the level of noise from their players and staff in the tunnel at the Etihad Stadium after Guardiola’s side had scored a 96th-minute winner to beat them.
It was at the end of that match when Guardiola had his animated on-pitch discussion with Saints player Nathan Redmond.
Meanwhile ,United midfielder Ander Herrera has called on the Premier League to change its rules and allow players to challenge bookings.
The Spaniard was cautioned for diving after going down easily following a challenge from Nicolas Otamendi, but claimed Oliver got it wrong and feels he should be able to appeal.
“I think the penalty is amazing,” said Herrera. “He [Otamendi] didn’t expect to arrive for the ball. I touched the ball first and he stamped on my foot. I think everyone saw it.
“Everyone can make mistakes, also the referee, but I didn’t deserve the yellow card.
“Now that’s my fourth yellow card and that’s a big problem for me because I want to play every game.
“In Spain, if you get something undeserved, you can appeal and they take it out.
“That’s one of the things that could improve the Premier League because it’s amazing competition.”