The doping case involving Greek sprinters Kostas Kenteris and Katerina Thanou has been postponed.
The case had been due to reconvene on 22 February
A Court of Arbitration for Sport (Cas) panel was due to continue hearing the case on 22 and 23 February.
But the panel's president, Yves Fortier, has resigned following allegations of a conflict of interest.
A Cas statement read: "He has decided to resign in order that the arbitration procedure could go on in an atmosphere of confidence and serenity."
The statement added: "Mr Fortier firmly rejected the allegations made against him and reiterated his full independence in this matter.
"While regretting this unfortunate situation, Cas notes Mr Fortier's decision and wishes to express publicly its entire confidence in an arbitrator whose independence and integrity as well as competence and dedication have for a long time been recognised."
Cas said it still hoped the arbitration could "be concluded in a reasonable time frame".
The composition of the Cas panel will now have to change and the hearing will either have to resume from the point it reached when it was adjourned last October or start from the beginning.
Kenteris and Thanou wanted Fortier to be removed as president of the panel because of an alleged conflict of interest.
He is a business partner of Stephen Drymer, who has worked for the World Anti-Doping Agency (Wada).
And Drymer was a clerk in the Greek sprinters' case before resigning.
The International Council of Arbitration for Sport was considering the challenge before Fortier resigned.
Gregory Ioannidis, who represents Kenteris and Thanou, said: "Although this is an extremely unfortunate development, we accept Mr Fortier's resignation.
"I do not wish to comment further on this issue."
Last week he told BBC Sport: "Mr Fortier is the president of the law firm that represents Wada in the investigation against Lance Armstrong.
"Mr Drymer was the ad hoc clerk in my clients' case. He decided to resign from his post as a result of the concerns expressed by my clients.
"Mr Kenteris and Ms Thanou are seriously concerned with this unfortunate development. The integrity of the process must be safeguarded at all costs."
But Cas general secretary Matthieu Reeb countered that he had "full confidence in the independence and impartiality of Yves Fortier".
"He (Fortier) has no relationship personally with Wada and in any case, Wada was not involved in the Kenteris and Thanou case."
The Cas panel in the Kenteris and Thanou case is exactly the same one that recently banned US sprinter Tim Montgomery for two years.
Montgomery has subsequently asked for the ban to be annulled because of an alleged conflict of interest.
Kenteris and Thanou were provisionally suspended by athletics' governing body, the IAAF, in 2004 but the decision was subsequently overturned by an independent Greek tribunal.
The IAAF then appealed to Cas, which will have the final say on the matter.