Ukraine has barred Russia's Eurovision contestant because of a visit she made to Crimea, annexed by Moscow in 2014.
Julia Samoilova was banned from Ukraine for three years because she had entered Crimea "illegally" - entering directly from Russia, not via Ukraine.
Samoilova, 27, has confirmed that she sang in Crimea in 2015. Ukraine is due to host Eurovision in Kiev, in May.
The European Broadcasting Union (EBU), which founded Eurovision, said it was "deeply disappointed" by the news.
"We have to respect the local laws of the host country, however we are deeply disappointed in this decision as we feel it goes against both the spirit of the contest and the notion of inclusivity that lies at the heart of its values," it said in a statement.
The EBU said it would "continue a dialogue with the Ukrainian authorities with the aim of ensuring that all artists can perform" in Kiev.
Senior Russian politicians called for a boycott of the event.