Ukraine has barred Russia's Eurovision contestant because she performed in Crimea, annexed by Moscow in 2014.
Julia Samoilova was banned from entering Ukraine for three years because of her "illegal" visit, the Ukrainian SBU security service said.
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.
A senior Russian politician, Frants Klintsevich, was quoted by Ria-Novosti news agency as saying Russia would boycott future Eurovision contests if the organisers failed to stand up for the singer.
The Eurovision song contest just got political. Russia and Ukraine are now locking horns over Moscow's choice of competitor.
There is already a predictable chorus of fury here. A deputy foreign minister called the ruling outrageous, cynical and inhumane - no less. One MP is demanding that the contest is moved to another country while a second warns that Russia will boycott every Eurovision to come unless organisers intervene to resolve this.
Some suspect this is precisely what Moscow wanted when it chose Julia Samoilova, knowing that she had travelled to Crimea.
Adding to the howls of outrage here is the fact that the singer being banned has used a wheelchair from childhood and this year the slogan for Eurovision is celebrating diversity.