Ilarion said that Constantinople's decision to back what the Russian Orthodox Church considers a schismatic church is "illegal and canonically worthless", and effectively drives it into a schism.
Ukraine accuses the Russian Orthodox Church of wielding a pernicious influence on its soil, allowing itself to be used as a tool of the Kremlin to justify Russian expansionism and support of separatist rebels in eastern Ukraine.
The move comes days after the Synod of the Constantinople Patriarchate chose to eventually grant the so-called autocephaly to the Ukrainian Orthodox Church, thus making the clerical organization, which earlier enjoyed a broad autonomy within the Moscow Patriarchate, fully independent.
In particular, it lifted the anathema from the heads of the two non-canonical churches, who had been excommunicated by the Russian Orthodox Church for their schism, and reversed the 1686 decision which transferred the Kiev Metropolis to the Moscow Patriarchate.
Constantinople's decision seems to be serving the interests of the Ukrainian leadership rather than the Orthodox Christians living there.
The Russian Orthodox Church said on Monday it had chose to sever all relations with the Ecumenical Patriarchate of Constantinople in protest over its endorsement of Ukraine's request for an "autocephalous", or independent, church.
"No other decision could have been taken by our Holy Synod because the logic of all the actions taken recently by the Constantinople Patriarchate led to this". Now, as Constantinople is launched a procedure of granting independence to the Ukrainian Orthodox Church, such attacks might further intensify, some experts warn.
Ukrainian President Petro Poroshenko said the Church's independence went hand-in-hand with Ukrainian independence. "It's an issue of Ukrainian statehood".
The Moscow Patriarchate, which is aligned with the Russian Orthodox Church, long dominated in Ukraine but since the 1991 Soviet break-up has been challenged by a rival known as the Kiev Patriarchate.
He said that up to a 10 Moscow Patriarchate parishes in Ukraine have switched over to the Kyiv Patriarchate since 2014 when Moscow invaded the easternmost regions of Luhansk and Donetsk after taking over the Crimea in a war that has killed more than 10,400 people and displace an additional 1.6 million. 9 jointly by the Kyiv International Institute of Sociology, the Razumkov Center and the SOCIS Center for Social and Marketing Research; 17 percent said they identify with the Moscow Patriarchate in Ukraine. "All these decisions are unlawful and canonically void", Hilarion said, adding that "the Russian Orthodox Church does not recognize these decisions and will not follow them".