Mikheil Saakashvili, the former Georgian president, has been stripped of his Ukrainian citizenship, Ukraine's migration service has said.
It said a decree was issued by Ukrainian President Petro Poroshenko, without stating the reason.
Mr Saakashvili, a former ally of Mr Poroshenko, is currently abroad. He vowed to fight for his rights.
He was made a Ukrainian citizen in 2015 and appointed governor of the southern Odessa region. He quit last year.
In a statement (in Ukrainian) on Wednesday evening, Ukraine's migration service said that "according to the Constitution of Ukraine, the president takes decisions on who is stripped of Ukrainian citizenship based on the conclusions of the citizenship commission".
It did not provide the exact reason, but stated that this could be done if a Ukrainian national acquired citizenship of another country or submitted false documents.
President Poroshenko has so far made no public comments on the issue.
Meanwhile, Mr Saakashvili posted a video on his Facebook page, saying that he was "being subjected to the same approaches that are used by Ukraine's prosecutors or bureaucrats against regular Ukrainians, whose rights are spat upon"
"I have only one citizenship, that of Ukraine, and I will not be deprived of it! Now there is an attempt under way to force me to become a refugee. This will not happen!
"I will fight for my legal right to return to Ukraine!"
Mr Saakashvili automatically lost his Georgian citizenship when he was given the Ukrainian one.
In his video message, he also suggested that President Poroshenko had made a deal with the authorities in Tbilisi during his visit to the country several days ago.
Mr Saakashvili, who led Georgia from 2004 to 2013, is accused by the Georgian authorities of abuse of power - charges that he describes as politically motivated.
Last year, he resigned as the Odessa governor, expressing fury at rampant corruption in Ukraine.
He has since formed his Movement of New Forces political party, which urged its supporters to hold a protest rally in Kiev on Thursday.
More than 100 people later gathered on Independence Maidan - the square in the heart of the capital, Ukrainska Pravda website reports.