Zurich - FIFA contender Prince Ali bin al Hussein said the international sports tribunal should delay the world football leadership election on Friday if it cannot be guaranteed to be "free, fair and honest".
The Court of Arbitration for Sport (CAS) said it would rule by Thursday, the eve of the vote, whether to order FIFA to change the organisation of the vote.
The prince has been joined by Jerome Champagne, another of the five contenders to replace Sepp Blatter, in demanding greater guarantees over the integrity of the increasingly tense election.
Sheikh Salman bin Ebrahim Al-Khalifa and Gianni Infantino are favourites in the race to become the next leader of the scandal-tainted world governing body. Tokyo Sexwale of South Africa is the fifth candidate.
Friday's vote is meant to be secret. But Prince Ali and Champagne have both raised suspicions that FIFA delegates - under pressure from some regional confederations who have publicly backed specific candidates - will take photos of their voting paper to prove who they signed for.
FIFA's electoral committee said portable phones and cameras would be banned from the booths, but it had rejected the prince's initial demand for transparent booths.
CAS has called on FIFA to explain its rejection of the prince's requests. Prince Ali has also demanded that independent scrutinizers are used for the vote.
The Lausanne-based tribunal said the prince has asked for "urgent provisional measures" but did not say whether a vote suspension could be ordered.
After Prince Ali's lawyers said he wants CAS to delay the election if transparent booths are not used, the candidate sent a letter himself to voters to clarify his position.
"The FIFA presidential election should go ahead as planned but only if it is free, fair and honest," he said.
Prince Ali described FIFA's plan to tell delegates to leave their phones outside as "unenforceable and without sanction".
"I want this election to occur as soon as possible, but not at the risk of undermining its integrity," he added.
Prince Ali said he had written to the FIFA Electoral Committee on February 11 to express his concerns that pressure would be put on delegates.
He said he offered to provide transparent booths as a solution to that problem but that FIFA rejected his idea.
"On February 22nd, I appealed to CAS and requested an expedited hearing to resolve the matter before Friday's election. FIFA blocked an expedited hearing," said Prince Ali.
With FIFA in desperate need of a clean start and reforms to improve its tainted image, the complaints have again cast a new shadow over the election campaign.
Champagne, a former FIFA official from France, has separately demanded that the world body cancel accreditations for the Asian Football Confederation and UEFA, saying they will be used to lobby for Sheikh Salman and Infantino.
He has indicated he could also go to CAS if FIFA's electoral commission rejected his request.
But electoral commission chairman Domenico Scala wrote to Champagne saying "FIFA has acted in absolute compliance with the pertinent circumstances" for the election.
Scala added that the observer numbers do "not constitute any violation of the principle of equal treatment of the FIFA presidential candidates".
Champagne accused Sheikh Salman, the AFC president, and Infantino, UEFA general secretary, of seeking to "swamp" Friday's vote with supporters.
He added that those accredited were mainly members of Sheikh Salman and Infantino's campaign teams.
"It is clear that this reveals the objective to swamp the Congress hall with confederation employees able to access the voting FAs and their delegates."
Champagne called on the electoral committee to cancel "these unfair and undue privileges" and warned of other action if there was no response by Tuesday.
FIFA has 209 member associations but only 207 are eligible to vote for a new leader.
Kuwait and Indonesia are suspended from the decision on who should lead the organisation as it seeks to recover from multiple scandals that have seen 39 football officials and business executives charged with corruption by US authorities. Two companies also face charges.
Swiss prosecutors are, in parallel, investigating FIFA's management and the attribution of the 2018 and 2022 World Cups to Russia and Qatar.
After Blatter announced he was standing down, UEFA leader Michel Platini had been favourite to take over.
But Blatter and Platini were suspended for eight years by FIFA over a $2 million payment that Blatter approved for the French football legend. That stunning move threw the election race wide open between Sheikh Salman and Infantino.