Benfica's starlet is at the centre of a threeway fight worth £120 million, according to The Mirror.
It's reported that both City and United will have scouts watching Felix in Portugal's Nations League semi-final clash against Switzerland in Porto on Wednesday.
Ole Gunnar Solskjaer has identified Felix as United's top priority and believed he could be the player to follow in the footsteps of ex-United star and compatriot Cristiano Ronaldo.
Pep Guardiola is also a huge admirer and is thought to believe he would fit perfectly into City's system.
However, it is Real Madrid that are leading the chase currently, having already submitted an £80 million bid, which was rejected.
Felix is represented by super-agent Jorge Mendes, who represents players at all three of the interested parties.
The 19-year-old striker has scored 18 goals in 33 games for Benfica this season and is set to earn his first senior cap for Portugal this week.
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