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"Russia was responsible for assassination of Aleksandr Litvinenko in the UK," the court said in a statement on its ruling on Tuesday.
Russia has always denied any involvement in Litvinenko's death.
Litvinenko (43), an outspoken critic of Russian President Vladimir Putin who fled Russia for Britain six years to the day before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel.
A British inquiry concluded in 2016 that Putin probably approved a Russian intelligence operation to murder Litvinenko.
It also found that former KGB bodyguard Andrei Lugovoy and another Russian, Dmitry Kovtun, carried out the killing as part of an operation probably directed by Russia’s Federal Security Service (FSB), the main successor to the Soviet-era KGB.
"The Court found in particular that there was a strong prima facie case that, in poisoning Mr Litvinenko, Mr Lugovoi and Mr Kovtun had been acting as agents of the Russian State," the European court said.