Relations between Moscow and Kiev have been locked in a downwards spiral since Russia annexed Crimea from Ukraine in 2014 and backed pro-Russian separatists in eastern Ukraine.
Poroshenko on Saturday signed a bill that orders the Ukrainian Orthodox Church to change its name to reflect its ties to Moscow.
The Ukrainian church has been part of the Russian Orthodox Church for centuries, but some Ukrainian orthodox clerics, with government support, have already moved to set up a separate independent Orthodox church in Ukraine.
Two weeks ago Ukraine started the process to establish its own, autonomous Orthodox Church, setting the stage for increased tensions with Russia by altering a centuries-old religious tradition under which the Kiev church answered to Moscow.
Poroshenko signed the bill ordering the Ukrainian Orthodox Church to explicitly indicate its link to the Moscow Patriarchate in its name.
He said that would make it easier for Orthodox believers to choose between the new independent church or an Orthodox church that is still tied to Moscow.
"It is easier to make a choice when all things are called by their names," Poroshenko said.
He insisted that the law doesn't infringe on the religious freedom. The Ukrainian Orthodox Church, however, has strongly protested the bill, denouncing it as an unconstitutional attempt by the government to meddle in the religious sphere.
When the Ukrainian parliament passed the bill on Thursday, over 1000 priests and parishioners of the Ukrainian Orthodox Church rallied outside the parliament building in Kiev to protest the move.
The Moscow Patriarchate also strongly condemned the creation of the new Ukrainian church, warning that it could trigger sectarian violence.
Metropolitan Hilarion at the Moscow Patriarchate said in televised comments on Saturday that the Ukrainian authorities' actions would encourage Ukrainian ultra-nationalists to try to grab the Ukrainian Orthodox Church's assets.
"They incite nationalist forces to continue their attempts to seize churches," Hilarion said.
The British defence minister, Gavin Williamson, visited Ukraine on Friday where he told his Ukrainian counterpart that the Black Sea did not belong to Russia and that Britain had sent a Royal Navy ship there to show Kiev did not stand alone.
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Williamson on Friday met Stepan Poltorak, his Ukrainian counterpart, and visited the port of Odessa where a British Royal Navy ship, HMS Echo, docked earlier this week.
"The reason ... that HMS Echo is here is that we firstly want to demonstrate the solidarity that we have with Ukraine and the fact that Ukraine doesn’t stand alone," said Williamson.
"But also demonstrate our right to be able to come to ports such as Odessa, for freedom of navigation, for freedom for navies to be able to operate in the Black Sea. This isn’t Russia’s sea, this is an international sea.”
Reuters, AP