One of the most bizarre rows between clubs broke out this week after Porto took aim at Manchester City in a newsletter to their fans following their 0-0 draw with Pep Guardiola's men in midweek.
In a baffling 529-word rant, the Portuguese side slammed City players and Guardiola for their behaviour during the game while celebrating their qualification to the Champions League knockouts after finishing as runner ups behind them in Group C.
Their decision to take this dramatic step of public criticism has more than raised a few eyebrows, and has resulted in City hitting back after their surprising words.
But where did this verbal spat come from and why is there an ill-feeling between the two sides? Sportsmail answers the key questions of this strange squabble.
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A bizarre rivalry is unfurling between Porto and Man City after their fiesty clash this week
How did it all start?
Well everything seemed fairly innocuous to begin with. You don't expect too many fireworks after a 0-0 stalemate - especially in a game that earned both sides qualification to the knockouts. Everyone should be happy, right?
Not quite. Porto had to settle for second place in the group in a game that City will claim they likely should have won. It took Guardiola's men a while to get going but Raheem Sterling had a shot cleared off the line just before half time.
Porto were happy to sit deep and hit City on the counter attack, with Sterling and Ferran Torres both going close after the interval - and they thought they had nicked it when Gabriel Jesus bundled home in the 80th minute, only to be judged as offside in the build up.
City boss Pep Guardiola took aim at Porto's defensive set-up after both sides drew 0-0
He said Porto had 'eight players in the box' after frustrating them at the Estadio do Dragao
Both sides settled for a point, but tension immediately started to bubble under the surface. Midfielder Fernandinho was the first to wade in - far from impressed with the opposition's conduct.Insurance Loans Mortgage Attorney Credit Lawyer
'It was a complicated game against a team with a unique style of playing,' he said. 'They put pressure on the referees, they fall from all fouls and it seems they need to call the ambulance.'
Meanwhile, Guardiola, was critical of Porto's defensive tactics under manager Sergio Conceicao as they held on for a point.
Porto coach Sergio Conceicao said he would be 'sad' if he failed to win with Pep's budget
'We attacked, we played a good game, but FC Porto had a lot of people behind. In this game, we had opportunities and did not score.
'They almost always had eight players in the area. Marega and Corona, they were also far behind.'
When asked for his thoughts on Guardiola's comments, Conceicao simply replied: ''If I had his budget, his players, and still couldn't get a win, I'd be sad too.'
What did Porto say?
They had a lot to say. The day after the game the club sent out a publication to their supporters which seemed like a run-of-the-mill summary of the game, until you get to the second paragraph.
It starts by commending the team after they were confirmed as 'one of the 16 best teams in Europe with a game to spare', before adding that they had won a point 'against the most expensive squad in the world'.
From there it turns very sour indeed - reminding fans of Conceicao's cutting words about Guardiola's failure to win, before taking their criticism even further.
Porto released a 529-word newsletter the following day criticising Guardiola and Man City
They also took exception to midfielder Fernandinho and claimed he should have been sent off
'The Manchester City coach and players - who also struggled to cope with the result - should even be grateful for the luck they had with refereeing again,' it read, claiming that their side should have had a penalty for Ederson's foul on Otavaio while slamming Fernandinho, who should've seen red in their eyes.
The Brazilian, they said was 'a clear example that football money counts a lot, but it doesn't buy class or notion'. This was a doubled-edged insult, aimed at the club and its spending habits - the third mention of the topic already.
City were left alone for a few more paragraphs while Porto boasted of how it had now qualified 16 times for the knockouts out of a possible 25 - far more than any of its Portuguese rivals Benfica or Sporting Lisbon.
Bernardo Silva was accused of racism as they referenced a social media post he made last year
Last November, Silva was fined £50,000 & banned for a game for a tweet to Benjamin Mendy as he compared his team-mate to the character on a packet of Conguitos
But they were quickly back on the assault again - this time hitting out at TV broadcasters in Portugal for criticising the team and singling one out for offering a shirt to an opponent of ours who is known internationally for having been convicted of racism' - referring to Bernardo Silva.
The Portuguese star, who started his career with Porto's bitter rivals Benfica, last year compared team-mate Benjamin Mendy to the character on a packet of Conguitos - a chocolate brand in Spain and Portugal. He was fined £50,000 and banned for one match by an FA panel, who acknowledged that he did not mean to cause offence.
It was a striking end to an explosive rant, one that would have left City furious without racism accusations.
How did Manchester City respond?
The fact they offered any kind of retort speaks volumes. It is not in the best interest of clubs to be seen giving the time of day to outbursts such as these, and City are renowned for keeping tight-lipped on most matters.
So their response was highly unusual and highlights the tensions between the two clubs. City clearly felt Porto had crossed the line and that something had to be said.
In a briefer and more measured statement, City hit back at the 'ill-judged' criticism that had been scattered around the team.
'This is not the first time that Porto have reacted badly in circumstances like this.
Manchester City rarely make public statements and their retort highlighted the tensions
They refuted Porto's criticisms pointed out how they had denied racist behaviour of Porto's fans towards Mario Balotelli (C) and Yaya Toure in 2012
'On this occasion it is the ill-judged and targeted criticism of some of our individual players and indeed our manager,