The European Court of Human Rights (ECHR) has ruled that Russia was responsible for the "assassination" of former Russian security officer Aleksandr Litvinenko in London in 2006.
Litvinenko, 43, an outspoken critic of Russian President Vladimir Putin, fell ill on November 1, 2006, after he drank tea that was poisoned with polonium-210, a rare, highly radioactive isotope.
He died several days later in the hospital.
A British inquiry concluded in 2016 that Putin probably approved a Russian intelligence operation to murder Litvinenko.
It also identified two Russians -- Andrei Lugovoi, now a Russian lawmaker, and Dmitry Kovtun -- as the primary suspects.
In its ruling issued on September 21, the Strasbourg-based ECHR said it had established "beyond reasonable doubt" that the killing had been carried out by Lugovoi and Kovtun, and 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."
Both men and Russia have denied any involvement in Litvinenko's death.
The Kremlin rejected the court ruling, calling it "unsubstantiated."
"The ECHR hardly has the authority or technological capacity to possess information on the matter. There are still no results from this investigation and making such claims is, at the very least, unsubstantiated," Kremlin spokesman Dmitry Peskov said.
Reuters quoted Lugovoi as saying the ECHR ruling was "absolutely politically motivated."
Litvinenko, a former officer in Russia's Federal Security Service (FSB), fled Russia in 1999 after revealing an alleged plan by the FSB to kill tycoon Boris Berezovsky.
Berezovsky had been prominent among the group of Russian businessmen known as the oligarchs who grew rich from the privatization of state assets following the collapse of Soviet communism.
As a Kremlin insider under former Russian President Boris Yeltsin, Berezovsky helped Putin rise to succeed his predecessor in 2000.
But when Putin moved to curb the political powers of the oligarchs, Berezovsky left Russia for self-imposed exile in Britain, where his criticism of Putin grew.
In 2006, the year Litvinenko was poisoned in London, Putin signed a law authorizing security agencies to carry out targeted assassinations abroad.
In January 2016, British Judge Robert Owen, who led an inquiry into Litvinenko's death, said it was probable that Putin, a former Soviet KGB officer who once headed the FSB, had approved the operation.
Owen said he was certain Lugovoi and Kovtun killed Litvinenko by placing the lethal dose of polonium-210 in his tea during a meeting at a London hotel.
British investigators have found traces of polonium-210 in hotels, restaurants, and aircraft used by Lugovoi, who was reportedly treated for radiation poisoning in Moscow in December 2006.
Lugovoi was elected a member of the Russian State Duma in 2007 and Putin awarded him a state medal "for services to the fatherland" in 2015.