Moscow's Patriarchate bishop has suggested postponing discussions about differences in the first meeting of its kind in more than a millennium, because of reports that other churches will not be attending.
The meeting is due to start in Crete, a Greek island, on Sunday.
There are about 100 million members of the Russian Orthodox Church.
The Orthodox churches are independent and have their own leadership, unlike other denominations, such as the Roman Catholic Church.
Orthodox leaders have not had a meeting since 787AD.
A "great schism" divided the Roman Catholics and the Orthodox in 1054 amid disputes over the Vatican's power.
However, now, the Ecumenical patriarch has tried to encourage discussion that could prevent the meeting from falling through.
Metropolitan Hilarion, head of the the Moscow Patriarchate's department of external relations, said that Russia would not be attending if others walked out.
It has now been hinted that the meeting could be postponed.
Hilarion said: "We have made a decision that we will not be able to take part in the all-Orthodox Synod if other churches do not go."
This move is damming for Istanbul-based Ecumenical patriarch Bartholomew I who has worked to stage the meeting and had hoped it would unify Orthodox churches throughout the world.
The Moscow Patriarchate has proposed holding a preliminary meeting to discuss differences before the main meeting for 14 independent Orthodox churches.
For example, Bulgaria's Orthodox church said last week that it would not attend, blaming differences over the agenda.
The first meeting will be held on 17th June, according to Revd Alexander Karloutsos, member of the Great and Holy Council.
"There is no mandate to change or postpone and we are going to proceed.
"They asked for a pre-conciliar meeting on the 17th and we expect to them to be here.
"We are the church of love and embrace everyone," he said.
The Russian Orthodox Church has said it will miss the Crete gathering if Bartholomew does not agree to reschedule.
It said: "All churches should take part in the all-Orthodox Synod and only in this case the decisions of this assembly will be legitimate."
The churches are due to discuss issues relating how the church functions as well as relations with other Christian denominations and the unity of the Orthodox churches being prerequisite to any reconciliation with the Vatican.
