The Russian Orthodox Church has rejected a last minute appeal to attend what was billed as the first meeting of fellow Church leaders since 787.
Russia says it cannot participate in the Holy and Great Council, on the Greek island of Crete, as not all churches will be present.
The Churches of Antioch, Bulgaria and Georgia had previously refused to take part after disputes about the meeting.
The gathering, due to start on Sunday, has been 55 years in preparation.
Fourteen Churches representing over 300 million faithful had been originally invited.
Experts say the decision by the Russian Church, which represents some 100 millon followers, highlights longstanding divisions among Orthodox Christians.
There is also a struggle for power between Russia and the Istanbul-based Ecumenical Patriarch of Constantinople, Bartholomew I, considered the spiritual head and “first among equals”.
He is the driving force behind the meeting.
The disagreements ranged from seating plans to efforts to reconcile with the Vatican.
Moscow also fears that Patriarch Bartholomew will give support to the Ukrainian branch of the Church, fuelling what one Russian state-owned newspaper has called “a simmering religious war”.
Russian Patriarch Kirill said he considered the Crete gathering a preparatory session for a synod that will unite all the Churches “without exception.”
- There are 300 million Orthodox Christians, who are members of 14 national Churches
- The denomination split from western Christianity – in the form of what is now Roman Catholicism – in 1054 amid disputes about the power of the Vatican
- Unlike the Roman Catholics, the Orthodox Churches are independent and have their own leadership
- Orthodox clergy are distinguished by their elaborate headgear and bushy facial hair, which they wear because they take literally some of the words of the Old Testament, which prohibit shaving