The stern warning follows Ecumenical Patriarch Bartholomew I's promise to allow the Orthodox Church in Ukraine to be autocephalous, or ecclesiastically independent.
The Russian church, the world's largest Orthodox communion, fiercely opposes the decision by Bartholomew, who is considered the "first among equals" of Orthodox leaders.
Moscow Patriarchate spokesman Vladimir Legoyda warned on Friday it will "break the Eucharistic communion" with the Istanbul-based Ecumenical Patriarchate if it makes the Ukrainian church autocephalous.
The church in Ukraine has been tied to the Moscow Patriarchate for hundreds of years, although many parishes have split off over the past two decades to form a schismatic church. Patriarch Kirill (pictured above) is currently the Russian Patriarch.
Calls for independence have increased since Moscow's 2014 annexation of the Crimean Peninsula from Ukraine.
Mr Legoyda said that the plans for autocephaly "threaten a fragile religious peace in Ukraine", and charged that they have been driven by "political ambitions of the Ukrainian leaders".
Ukrainian President Petro Poroshenko, who is running for re-election next March, has strongly pushed Bartholomew (pictured above with then-US President Barack Obama) to grant independence to the Ukrainian church.
His efforts received a fillip earlier this month when the Ecumenical Patriarchate announced that it was sending two bishops to Ukraine as a step toward declaring ecclesiastical independence for the church there.
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