By Jan Repa
BBC Europe analyst
In a move that could embarrass the Vatican, the Ukrainian Greek Catholic Church has announced it is moving its headquarters on Sunday from the western Ukrainian city of Lviv to the country's capital, Kiev.
Kiev is regarded as one of the major centres of the Orthodox faith
Originally part of the Orthodox Church, the Greek-rite Catholics, or "Uniates", recognised the Pope as the head of their church several hundred years ago.
The Vatican appears in two minds about the Uniates' assertiveness - being anxious not to further antagonise the Orthodox churches, with which is trying to improve relations.
The Uniate Church claims five to six million adherents - mainly in western Ukraine - plus a further 1.5m among ethnic Ukrainians living in Russia.
The Church was originally established in 1596, when most of today's Ukraine was part of Catholic Poland.
It was agreed that the Uniates would keep their traditional Orthodox rituals - and practices like a married priesthood - but would become part of the Catholic Church.
The "Union" was controversial from the start. Orthodox Christians saw it as an annexation of part of their own church by Rome.
Cardinal Husar dismisses objections from Moscow as "irrelevant"
With the expansion of the Orthodox Russian empire into Ukraine, in the 17th and 18th centuries, the Uniate Church was banned.
It survived mainly in those parts of the old Polish state which were incorporated into the Catholic Austrian Empire in 1772. These areas, eventually annexed by the Soviet Union during World War II, now form part of western Ukraine.
In deciding to move its headquarters from western Lviv to Kiev, the Uniate Church is indicating that it sees itself as a pan-Ukrainian church - rather than a regional peculiarity.
The Uniates also want to upgrade their headquarters to the level of a Patriarchate - nominally the equal of the various national Orthodox churches - Russia's included - but still under the authority of the Pope in Rome.
This has set alarm bells ringing throughout the Orthodox world, as Kiev is regarded as one of the major centres of the Orthodox faith and the ancestral city of Russians, Belarussians and Ukrainians.
Patriarch Alexy II (right) sees the move as an "unfriendly" act
A Kiev-based group, calling itself the Union of Orthodox Citizens, has declared that it would "defend Kiev as the mother of Russian cities and New Jerusalem - which cannot exist without the Third Rome (Moscow)".
The Russian Orthodox Church says the Uniates' plans are "uncanonical" - or contrary to Church law.
The head of the Ukrainian Uniates, Cardinal Lubomyr Husar, says such objections are "irrelevant", because the Uniates and Orthodox belong to two different churches.
Religion and nationalism have long gone hand-in-hand in Eastern Europe.
During last year's "Orange Revolution" - when pro-Western presidential candidate Viktor Yushchenko defeated Moscow-backed Viktor Yanukovych - the Ukrainian branch of the Russian Orthodox Church agitated in favour of Mr Yanukovych, while the Uniate Church openly declared for Mr Yushchenko.
According to Vatican sources, Pope Benedict XVI shares the opinion of his predecessor, the late John Paul II, that in view of the persecution the Uniate Church has suffered, its ambitions are understandable - and will have to be addressed sooner or later.
However, the Vatican also prefers a cautious approach and does not like being "bounced" by the Uniate leadership.
There is, on the face of it, not much the Vatican can do about the planned move to Kiev on 21 August.
However, giving the Uniates a Patriarchate is another matter.