Guerrero, Peru's all-time top scorer, was suspended for a year by Federation Internationale de Football Association on December 8 after testing positive for cocaine metabolite benzoylecgonine following Peru's 0-0 draw away to Argentina the previous October, after apparently drinking tea containing the substance.
Peru, who have made it to the World Cup qualification for the first time since dating way back in 1982, will be playing the group stage round with Australia, Denmark and France in Group C in Russian Federation.
The doping ban that will prevent Peru captain Paolo Guerrero of his long-awaited chance to play at the World Cup is unfair and defies common sense, the world players' union FIFPro said on Tuesday.
The 34-year-old footballer is in Peru's provisional squad that was named on Sunday.
Guerrero, who has denied wrongdoing throughout, had also appealed to CAS to request the ban be annulled altogether.
The forward was originally banned from all FIFA competitions for a year, which would have kept him out of Russia had the ban not been reduced by FIFA, after an appeal, to six months.
Guerrero, whose lawyers argued that the Benzoylecgonine was accidentally consumed in tea, plays his club football for Brazilian outfit Flamengo.
The CAS panel confirmed the existence of an ADRV [anti-doping rule violation] committed by Mr Guerrero, but also accepted that he did not attempt to enhance his performance by ingesting the prohibited substance.
Guerrero has 32 goals for his country in 86 appearances.
CAS accepted he did not intend to enhance his performance but ruled that Guerrero bore "some fault or negligence, even if it was not significant".
After the match, Guerrero said: "There is no reason for me to be suspended, I did nothing wrong and I trust the court to make the right decision".
This is the second ruling by CAS that has a huge impact on Peru's 2018 World Cup journey. The three points helped Peru qualify for the playoffs- where they beat New Zealand to make it to Russia- at the expense of Chile.
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