Wladimir Klitschko may pull out of fighting Alexander Potevkin over drug testing, the Ukrainian's manager has warned.
The WBO, IBF and IBO, WBA 'super' champion was due to face the WBA's 'regular' champion in Moscow on October 5 - but the camp are insisting Germany's National Anti-Doping Agency (NADA) handle any drugs testing.
Povetkin, meanwhile, wants to use a Russian agency - and Klitschko's manager Bernd Boente has confirmed that the disagreement may result in the bout being called off altogether.
He said: "There are still huge gaps in the contracts, not only in terms of the drugs testing.
"Better safe than sorry. I've already seen too much in this business."
The fight has been funded by a Russian promoter in Vladimir Hryunov, who won the purse bid to stage the long-awaited clash with a staggering 18m euros.
Klitschko, 37, and Povetkin, 33 - both Olympic champions - had been due to meet in 2008 and 2010 but on both occasions, the Russian withdrew.