ISTANBUL (Reuters) - The spiritual head of Orthodox Christians worldwide formally granted independence to the Ukrainian church on Saturday, marking an historic split from Russia which Ukrainian leaders see as vital to the country’s security.

The decree, granting “autocephaly”, was signed by Ecumenical Patriarch Bartholomew at a service with the head of the Ukrainian church Metropolitan Epifaniy and President Petro Poroshenko in St George’s Cathedral in Istanbul.

“I want to thank the millions of Ukrainians around the world who responded to my appeal to pray for the church to be established,” Poroshenko said at a ceremony accompanied by solemn liturgical singing.

“I want to thank the generations of Ukrainians who dreamed...and finally God sent us the Orthodox Church of Ukraine,” he told the congregation in the crowded church.

The patriarchate, the seat of the spiritual leader of some 300 million Orthodox Christians worldwide, endorsed Ukraine’s request for the new church in October. The decree, or Tomos, will be handed to Epifaniy at a ceremony on Sunday, completing the process of recognition by the Ecumenical Patriarchate.

Ukraine last month chose 39-year-old Epifaniy to head the new church, in a move which Poroshenko compared to Ukraine’s referendum for independence from the Soviet Union in 1991.

The move incensed Moscow, and prompted President Vladimir Putin to warn of possible bloodshed in his annual news conference. Relations between Ukraine and Russia collapsed after Moscow’s seizure of Crimea in 2014.

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Ukraine imposed martial law in November, citing the threat of a full-scale invasion after Russia captured three of its vessels in the Kerch Strait.

BITTER RUSSIAN OPPOSITION

The Ukrainian Orthodox church has been beholden to Moscow for hundreds of years, and Ukraine’s leaders see church independence as vital to tackling Russian meddling.

Kiev says Moscow-backed churches on its soil are a Kremlin tool to spread propaganda and support fighters in the eastern Donbass region in a conflict that has killed more than 10,000 people. The churches strongly deny this.

“Tomos - is just a paper, the result of restless political and personal ambitions. It was signed in breach of canonicity and this is why it has no power”, Vladimir Legoida, the head of the Russian Orthodox Church Synodal Department for Church-Society and Media Relations, posted in Telegram messenger.

Epifaniy was chosen by a council at the St Sophia Cathedral in Kiev, built by the son of Prince Volodymyr whose baptism in 988 led to the spread of Christianity in the region.

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The new church may boost pro-Western leader Poroshenko, who lobbied hard for its creation and faces a tight election race in March.

Russia bitterly opposes the split, comparing it to the Great Schism of 1054 that divided western and eastern Christianity. Russian Orthodox Patriarch Kirill made a last ditch appeal against the process last month.

“Huge win for Ukraine, defeat for the Kremlin,” economist Timothy Ash wrote on Twitter. “(It) will make Moscow’s hope of some future pull of Ukraine back into its ‘orbit’ nigh on impossible without the use of overwhelming (catastrophic) military force.”

Religious divisions deepened in Ukraine after 2014 and two Orthodox factions vie for dominance.

The church known as the Moscow Patriarchate, aligned with the Russian Orthodox Church, sees itself as the only legitimate church in Ukraine. On Dec. 20, Ukrainian MPs passed a law that could force the church to add “Russian” to its name.

The rival Kiev Patriarchate was born after the collapse of the Soviet Union and its popularity has grown since 2014. It favors European integration and championed the independent church but the Moscow Patriarchate denounces it as schismatic.