UEFA's chief investigator Yves Leterme refused to communicate with Manchester City regarding his probe into claims of Financial Fair Play breaches.
The exchanges were revealed after the publication of the Court of Arbitration's ruling that the case against City should proceed despite the club's objection.
City had claimed that leaks to certain media outlets predicting the referral of the case from the Club Financial Control Body Investigatory Chamber to the Adjudicatory Chamber meant the case should be halted, reports the Daily Mail.
City wrote to the CFCB to protest but Leterme wrote back: "I must vehemently reject your allegations of unlawful activities, either by myself or by any of the members of the Uefa CFCB... Your allegations are groundless and unacceptable in tone."
City also cited an interview Leterme gave to magazine Sport & Strategy in 2018, shortly after the hacked emails had been published by German magazine Der Spiegel.
"If it is true what has been written, there might be a serious problem," said Leterme. "This can lead to the heaviest punishment: exclusion from the Uefa competitions."