(Refiles to clarify description of Russia-Georgia conflict in paragraph 3)

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KIEV (Reuters) - Ukrainian President Petro Poroshenko has stripped one-time ally Mikheil Saakashvili of his Ukrainian citizenship, the migration service said on Wednesday, spelling the likely end of the former Georgian president’s political aspirations in Ukraine.

Saakashvili, who is widely credited with cracking down on graft as president of Georgia from 2004 till 2013, was given a Ukrainian passport and appointed governor of Ukraine’s southern Odessa region in a surprise move by Poroshenko in May 2015.

Saakashvili, whose country fought a war with Russia in 2008 and who is despised by the Kremlin, has seemed a natural ally for Poroshenko whose country lost Crimea to annexation by Russia and is fighting a war against Moscow-backed separatists.

However relations between the two men soured and Saakashvili resigned as governor last November to establish a new political party in opposition to Poroshenko, whom he accused of allowing corruption to flourish.

The migration service did not state the exact reason for Saakashvili’s loss of citizenship, though it said he may have acquired citizenship of another country or submitted false documents.

“According to the constitution of Ukraine, the President of Ukraine takes decisions on losses of Ukrainian citizenship based on the conclusions of the citizenship commission,” it said in a statement.

Saakashvili did not immediately respond to a request for comment.

Saakashvili’s party, ‘Movement of New Forces,’ (MNF) said if correct, the withdrawal showed that Poroshenko had chosen “a path of anti-constitutional acts for the sake of the usurpation and retention of power at all costs.”

In a post on Facebook, the party accused Poroshenko of betraying the reformist aims of the 2013-2014 pro-European ‘Maidan’ uprising that helped bring him to power.

Reformist lawmaker Mustafa Nayyem, who is not a member of MNF, condemned the decision to withdraw Saakashvili’s citizenship.

“This is the stupidest thing that could have been thought up - a sign of weakness that reflects badly on the president of Ukraine,” he said in a post on Facebook that was shared by Saakashvili.

According to Ukrainian legislation, only Ukrainian citizens can lead political parties or be elected to parliament.

MNF has been campaigning for early parliamentary elections. According to a June survey by Ukrainian group Rating, 2 percent of Ukrainians would back the party, while 6 percent favour Poroshenko’s bloc.