The Russian Orthodox Church on Saturday threatened to retaliate against its Istanbul-based rival if it allows Ukraine to cut spiritual ties with Moscow, ending Russian religious rule in the country.
Istanbul-based Patriarch Bartholomew of Constantinople, known as the "first among equals" of Orthodox Christian leaders, is expected to rule in coming months on the Ukrainian appeal to break away from Moscow by creating an independent church.
But top Russian church official Metropolitan Illarion on Saturday said that if the patriarch of Kiev was recognised "we will have no choice but to sever relations with Constantinople", accusing Bartholomew of acting in a "despicable and treacherous way".
"When one brazenly and cynically interferes in the affairs of a local Orthodox Church, one creates not only a hopeless situation, but threatens the global Orthodox world with a schism," Russian news agencies quoted him as saying.
Bartholomew's rival, Russia's Patriarch Kirill, has called the prospect of the Ukrainian church being separated from Moscow an "all-Orthodox catastrophe".
The Orthodox church in Ukraine is split between a branch whose clerics pledge loyalty to Moscow and one that is overseen by the unrecognised Kiev-based Patriarch Filaret.
While Constantinople is the oldest Orthodox Church, Moscow is currently the most powerful with the largest number of worshippers.
It is unclear what granting Ukraine the right to create an independent church will mean in practice.
But experts agree that, whatever shape such a decision would take, it would be a blow to Russia's spiritual authority in the Orthodox world.
The row comes against the backdrop of an ongoing, four-year conflict between Kiev and Russian-backed rebels in eastern Ukraine that made many Ukrainians turn away from the Moscow church.