World Anti-Doping Agency (Wada) vice-president Linda Helleland says she will vote against lifting the suspension of Russia's anti-doping agency (Rusada) this week.
Wada's executive committee meets in the Seychelles on Thursday where they are expected to pave the way for Russia's readmission into international sport after a major doping scandal.
But Helleland has become the first member of Wada's senior leadership to oppose the move, insisting the country has not yet met key demands.
"I will vote against the reinstatement of Russia," she said as the issue continued to cause unprecedented division within the organisation.
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Last week, Wada's compliance review committee (CRC) recommended that its executive committee end a three-year suspension of Rusada, saying the country had "sufficiently acknowledged" failures.
Wada had previously insisted Russia meet two criteria as part of a 'roadmap' for its return to compliance: accept the findings of the 2015 McLaren report after an investigation revealed an extensive, government-backed cheating conspiracy, and grant access to its drug-tainted Moscow laboratory.
However BBC Sport then revealed details of a compromise suggested by Wada's president Sir Craig Reedie and director-general Olivier Niggli to Russia's Sports Minister in June that was eventually accepted.
Wada defended the apparent softening of its position, but the revelation has sparked an outcry from various athletes and national anti-doping agencies.
Now Helleland - who hopes to replace Reedie as Wada president next year - has also broken ranks.
In a statement the Norwegian politician said: "I can see that progress is being made and I acknowledge the efforts done by Rusada, but as long as the McLaren report is not acknowledged and Wada still has no access to the laboratories, I will vote against the reinstatement of Russia.
"I am in no doubt that the tabled proposal is deviating considerably from the original roadmap and hence I feel I am obliged to defend previous decisions at the Wada ExCo (executive committee). This is one of the most critical decisions the anti-doping community has ever faced.
"I will vote for, and support the original roadmap. This is because I believe you should never make any compromises that undermine your credibility.
"If you choose to reinstate Russia, you defy the very wish of the athletes' committees around the world, who have very clearly stated that they will not accept a reinstatement now.
"This moment will forever define the credibility of Wada as the independent and strong front runner for clean sport."
Russia has repeatedly denied running a state-sponsored doping programme, and have been approached for comment.
In a letter to Reedie last week, Russian sports minister Pavel Kolobkov said: "I am grateful for your acknowledgement of the significant achievements in rebuilding Rusada."
And Reedie has defended the compromise, saying: "It's my convinced view that the best thing Wada can do is to have an efficient and well-operated anti-doping system in Russia and we've been trying to do that for two years. This moves things forward."
Calls for decision to be postponed
Meanwhile, seven members of Wada's athlete committee - from seven countries - have signed a statement in which they say "any compromise… will be a devastating blow to clean athletes and clean sport".
Ten members of the Wada athlete committee have not added their names to the statement, although its chair - Canadian Olympian Beckie Scott, who resigned from the CRC in protest - is also understood to be in support.
"It is for Rusada to be compliant, not for Wada to change its conditions to make Rusada compliant," they say.
"It should not be possible to commit the biggest doping scandal of the 21st Century and then be reinstated without completing the conditions that have been set.
"Any compromise on the road map will be a devastating blow to clean athletes and clean sport."
As the civil war in global anti-doping has intensified, the UK Anti-Doping Agency (Ukad) has joined with other leading national anti-doping organisations around the world to call for a postponement of any decision by Wada.
In a remarkable joint statement, the bodies said they were "dismayed" at what they called "a shifting of the goalposts" by Wada and accused it of "sending a message to the world that doping is tolerated" over its deal with Russia.
"In the interests of athletes we urge Wada to postpone the decision of its executive committee on Thursday until such time as Russia has clearly and publicly met the outstanding conditions of Wada's roadmap," they said.
Ukad boss Nicole Sapstead told the Press Association: "This just looks bad, so bad. What happened to accountability and transparency? I am very uncomfortable that athletes and the anti-doping community have been given such a short time to respond and react to something so important."
Meanwhile, the US Olympic Committee (USOC) has voiced its concern over the issue.
"We hope that Wada can reach a conclusion on Rusada that will give athletes a firm belief that when they compete, it will be on a level playing field, without any doubts," said chief executive Sarah Hirshland.
"Anything that stops short of satisfying that will not only be a huge disappointment to the USOC and American athletes, but to the entire Olympic and Paralympic movements."