By Henry Young, CNN

(CNN) -- It's a story involving an alleged drugs violation and Inca mummies that's enraged a South American nation about to make its first World Cup appearance since 1982.

Thousands have taken to the streets of Peru in protest and three rival national team captains have signed a letter calling for leniency in the case of the Peruvian forward Paolo Guerrero.

Peru's all-time top scorer is presently banned from the Russia 2018 World Cup after testing positive for the cocaine metabolite benzoylecgonine, following a World Cup qualifying match away to Argentina in October 2017.

Initially banned for a year by FIFA -- the substance is on the World Anti-Doping Agency's prohibited list -- Guerrero protested his innocence, contending the metabolite was only found in his system as a consequence of a simple cup of coca tea, a common drink in South America.

In court, Guerrero's lawyers invoked the story of three Inca mummies -- the Children of Llullaillaco -- discovered by mountaineers in 1999.

The mummies' perfectly-preserved bodies had gone undisturbed since the 16th Century and contained traces of the same substance -- despite cocaine only being isolated as psychoactive alkaloid hundreds of years later.

If the metabolite could hang around on a person for centuries before cocaine even existed, surely, the lawyers argued, an adverse finding in Guerrero's case wasn't so unusual.

The player posted on his Facebook page in November: "I trust that the truth will soon be known and I will return to the fields to defend with soul and heart the colors of my country."

On 20 December 2017, FIFA reduced Guerrero's ban to what it considered a "proportionate sanction" of six months, meaning Peru's talismanic captain would get a chance to play in the World Cup.

Job done, or so it appeared, as Peru manager Ricardo Gareca named the 34-year-old in his provisional squad on May 14.

The World Anti-Doping Agency Agency had other ideas.

Later that day, the Court of Arbitration for Sport (CAS) announced it had decided to increase the suspension from six to 14 months, as WADA sought to impose a stricter ruling.

CAS accepted that Guerrero "did not attempt to enhance his performance by ingesting the prohibited substance," adding there was no "significant fault or negligence," but the court's panel insisted the player could have taken measures to prevent the violation.

Guerrero had also taken his case to CAS, seeking to prove his innocence once and for all. The verdict left him back at square one.

Rivals join forces

It's a decision that has prompted uproar in his native country and beyond.

Newly-appointed president Martin Vizcarra vowed to support Guerrero's case, lobbying the Peruvian embassy in Switzerland.

Thousands crowded onto the streets of Lima donned in Peru's national colors with placards and banners protesting Guerrero's innocence.

FIFPro, the sports world players' union, called for an "urgent" meeting with FIFA, calling the ban "unfair and disproportionate."

Hugo Lloris, Mile Jedinak and Simon Kjaer -- captains of France, Australia and Denmark respectively -- signed a letter dated May 21 calling for FIFA to show "compassion" and "temporarily interrupt" the ban.

"Peru is returning to football's ultimate stage after a 36-year absence and we believe Paolo Guerrero should be allowed to lead his nation and celebrate what will be a career highlight," says the letter, addressed to the FIFA General Secretariat.

"We turn to the FIFA Council and kindly put forward an urgent request for clemency by asking the FIFA Council to temporarily interrupt the ban imposed on Paolo Guerrero during the 2018 World Cup in Russia, with the suspension to recommence at the conclusion of Peru's participation in the competition."

An audience with Infantino

Guerrero, nicknamed the "Warrior," certainly isn't going down without a fight.

On Tuesday, the striker and Peru Football Federation president Edwin Oviedo flew to Zurich for a personal meeting with FIFA president Gianni Infantino.

In a video published on his Facebook page, the player thanked the legions of Peruvians that supported his cause and said he aimed to return from Switzerland with "good news."

But, while Infantino expressed his "deep understanding of Guerrero's disappointment," a FIFA spokesperson told CNN the meeting would have "no impact" on the final decision, because the sanction had been imposed by CAS "after an appeal lodged against a decision of an independent FIFA judicial body."

They've "effectively drawn a blank for now," according to FIFPro's director of communications, Andrew Orsatti.

"It was always going to be a long shot," Orsatti told CNN Sport Wednesday, "because FIFA perhaps was not prepared to take on a ruling which is final and binding from the Court of Arbitration for Sport."

"There may still be a glimmer of hope, you never know. There's still some time to look at whether there are any technical errors in the legal procedure which might give rise to the idea of delaying the suspension until after the World Cup.

"FIFA still has a vested interest in looking at whether or not there is a solution here, because the public outcry has been enormous."

'Trapped in the pitfalls of the system'

Whatever happens to Guerrero, the wider implications of the case could be significant, according to FIFPro.

"The current anti-doping system requires reforms on different fronts," Director of Policy and Strategic Affairs, Jonas Baer-Hoffman, told CNN Tuesday.

"The last years have proven that the system is ineffective and at the same time very invasive to the rights of athletes, who too often get trapped in the unjust pitfalls of the system. Athletes should be the first stakeholder to build a system for a clean sport.

"We fail to see the purpose and benefit for our sport of such a sanction. The tribunals recognized there was no intent by Paolo Guerrero to cheat. Mandatory sanctions, with such severe consequences, do not help the fight against doping and rob a player of the pinnacle of his career."

Orsatti calls Guerrero a "sacrificial lamb," stressing the need for "a new collective bargaining agreement between WADA, the International Olympic Committee and FIFA, to make the rules fair and proportionate to the specifics of this sport.

"Unfortunately when it comes to change, reform and adjusting something which just isn't right, it always takes someone to suffer -- something of a sacrificial lamb," said Orsatti.

"In this case we're seeing Guerrero being the one being caught in the firing line. Even though you have no intent to cheat -- which everyone agrees around the case -- you are still banned according to the doping rules for one to two years."

FIFPro proposes an alternative anti-doping system that protects both the sport's integrity and the athletes going forward.

"Other sports especially in the United States have proven that in cooperation with the athletes, organized in their unions, effective, fair and strong anti-doping systems can be built," said Baer-Hoffman.

"We would like to see the start of a process with the other stakeholders to build a system which effectively protects sports' integrity and does not leave athletes exposed to situations as faced by Paolo Guerrero at this moment."

A WADA spokeperson told CNN the agency was in the midst of a "three-phase review process" of its rules, arguing CAS had delivered a decision "in line" with their current guidelines.

"One of the strengths of the Code is that it is constantly evolving and improving," said the WADA statement.

"In December 2017, WADA launched the first of a three-phase review process of the Code that will come to an end in November 2019 and which will also involve review of the International Standards. This is the third review of the Code, which ensures that the document continues to protect clean athletes effectively and fulfills the needs of the anti-doping community."

Any changes to the current rules will come far too late for Guerrero, with the final deadline for national squads set for June 4.

"WADA think there's a once-size-fits-all approach," said Orsatti. "Well a once-size-fits-all approach never suited anyone, and this is where the problem occurred.