Uefa has opened disciplinary proceedings against Porto for "improper conduct" of fans during the Europa League match against Manchester City.
The Premier League leaders made an official complaint to the European governing body following their first-leg
win in Portugal on 16 February.
City alleged there had been racial abuse towards striker Mario Balotelli and midfielder Yaya Toure.
A statement from Uefa said that it would deal with the case on 29 March.
The statement added: "Following a complaint lodged by Manchester City FC, Uefa has today opened disciplinary proceedings against FC Porto for the improper conduct of supporters (article 11 of the Uefa disciplinary regulations) during their Uefa Europa League round of 32 first-leg match against Manchester City FC last week."
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Porto coach Vitor Pereira
“What I have to say on that, is that normally in our league and our stadiums we don't have problems like that regarding racism”
Uefa had been waiting to see the report from its match delegate before deciding whether to launch a full investigation.
At the pre-match press conference before the return leg with City on Wednesday, Porto coach Vitor Pereira and striker Hulk claimed there might have been a misunderstanding over chanting in the first game.
The pair both said the home fans may have been chanting "Hulk, Hulk, Hulk" at the club's Brazilian player during the game.
"What I have to say on that, is that normally in our league and our stadiums we don't have problems like that regarding racism," said Pereira.
"In my opinion it was probably a misunderstanding and the authorities will deal with it. I am sure it will be sorted out."
Hulk said he did not hear any racial abuse directed at City's players.
"I did not notice any racist abuse, as the manager said. "In our stadium I always hear this, 'Hulk, Hulk, Hulk,' particularly as I [take] the corners," he said.
If found guilty, Porto face punishment ranging from playing a match behind closed doors to being banned from a Uefa competition, plus they may be fined.