Fifa's audit chief Domenico Scala has resigned in protest at a power grab by President Gianni Infantino over control of independent panels that monitor the scandal-hit football body.
Mr Scala's resignation on Saturday comes amid claims of tension between the two men over Mr Infantino's salary.
It marks the first major challenge to Mr Infantino's presidency since he was elected to succeed Sepp Blatter in February.
Mr Scala, who has monitored Fifa's billion-dollar annual spending since 2012, described his resignation as a "wake-up call" for people working to reform Fifa.
Tensions between the two men were exposed on Friday at Fifa's annual congress in Mexico City. Member federations voted in new powers to Mr Infantino's ruling council to fire Mr Scala and ethics committee leaders who investigate corruption claims.
Those independent officials have been seen as a key check on Fifa since their appointments were the main achievements of a Blatter-led round of anti-corruption reforms in 2012.
"I am consternated about this decision," Mr Scala said in a resignation statement, citing Friday's move, "because it undermines a central pillar of the good governance of Fifa, and it destroys a substantial achievement of the reforms".
In a speech on Friday, Mr Infantino declared Fifa's corruption-fuelled crisis to be over.
He later defended his new powers to remove key people overseeing his work when he was questioned at a news conference.
"Those that are making the comments have not really understood what we are doing," Mr Infantino said in Mexico City, adding his council would have the powers for only one year.
"The judgments need to be made by the quality of the members which are sitting on these boards rather than by making speculations or putting intentions in the minds of people which are far from the reality," he said.
But the tactic was criticised by former Fifa anti-corruption adviser Mark Pieth who helped bring Mr Scala, a Swiss pharmaceutical industry executive, to Fifa four years ago.
"(Infantino) is actually exactly working like (Michel) Platini and Blatter," Mr Pieth said. "We desperately want to go beyond that now."
Mr Pieth suggested that Mr Infantino disagreed over a salary offer of 2 million Swiss francs (£1.4m) made by a three-member Fifa remuneration panel headed by Mr Scala.
"It is personal, it is very clear," Pieth said. "He wants more than the 2 million that Domenico is offering him."