The Portuguese outfit criticised City manager Pep Guardiola, midfielder Fernandinho and referenced a year-old racism case involving playmaker Bernardo Silva in the official email.

City responded by calling the attacks "ill-judged".

The Dragoes Diario - Dragons Daily - also branded City as lucky after the goalless draw at the Estadio do Dragao, from which the hosts secured a place in the last 16 and the visitors claimed top spot in Group C.

The letter, which was seen and published by Portuguese newspaper A Bola, referred to Guardiola's "bad disposition" and then highlighted a quote criticising him from Porto head coach Sergio Conceicao.

"I would also be upset if I couldn't win with the team he has and the budget he has," Conceicao was quoted as saying.

This follows on from clashes between the two managers when the sides met at the Etihad Stadium in October.

The newsletter went on to accuse Fernandinho of lacking class. It said the Brazilian was "a clear example that football money counts a lot, but it doesn't buy class or notion". It added he should have been sent off in the match.

Silva, who previously played for Porto's rivals Benfica, was then referred to as someone "known internationally for having been convicted of racism".

This was a reference to a social media post by Silva last year in which he compared teammate Benjamin Mendy to the character on a packet of Conguitos chocolates.

He was banned for one game, although the Football Association accepted he did not intend to cause offence with the post.

This last remark has particularly upset City, who have highlighted the fact Porto were fined for the racist behaviour of their own fans following a controversial meeting between the clubs in February 2012.

On that occasion, in the Europa League, monkey chants were directed at Mario Balotelli and Yaya Toure from supporters at the Estadio do Dragao.

A City spokeperson said: "This is not the first time that Porto have reacted badly in circumstances like this. On this occasion it is the ill-judged and targeted criticism of some of our individual players and indeed our manager, which we wholly reject.

"In 2012, the last season we encountered them, it was the denial by the club of clear racist behaviour of their fans for which they were investigated and fined.

"In this context, this latest outburst it almost as surprising as it is disappointing."

The Porto newsletter also said City should "be grateful for the luck they had with refereeing" over some decisions which went the visitors' way.

The latest controversy adds to the bad blood that was evident between the two clubs in October when Conceicao accused Guardiola of attempting to influence the officials.

Australian Associated Press