The European Court of Human Rights in London has ruled Russia was responsible for ex-KBG officer Alexander Litvinenko's poisoning death.
The prominent Kremlic critic and whistleblower died in the agonising polonium poisoning incident in the UK in 2006.
Pictures of a desperately ill Litvinenko on his deathbed shocked the world.
Scientists uncovered the 43-year-old had been poisoned at a plush London hotel after drinking green tea laced with Polonium 210, a rare radioactive isotope.
After moving to Britain, Litvinenko, a former FSB officer, had become a vocal critic of Russian President Vladimir Putin.
Moscow had always denied a hand in his death. But on Tuesday, the Russian state was finally officially named the culprit five years after a UK inquiry pointed the finger of blame.
"Russia was responsible for assassination of Aleksandr Litvinenko in the UK," the human rights court said in a statement on its ruling.
The court found Litvinenko's assassination was "imputable to Russia".
Russia has always denied any involvement in Litvinenko's death which plunged Anglo-Russian relations to a post-Cold War low.
Litvinenko fled with his family to London after he and colleagues publicly accused their superiors of ordering the assassination of oligarch Boris Berezovsky.
He was granted asylum in the UK, where he worked as a journalist and consultant to Britain's spy agencies.
He wrote books accusing the Russian state and Putin himself of ordering an assassination and a false-flag terror attack to cultivate support for the leader's rise to power.
On November 1, 2006, Litvinenko suddenly fell ill and was hospitalised with a case of polonium poisoning.
He died 22 days later in London's University College Hospital.
Before he died, Litvinenko said: "You may succeed in silencing one man but the howl of protest from around the world, Mr Putin, will reverberate in your ears for the rest of your life."
His widow, Maria Litvinenko, accused Moscow of orchestrating the murder, though she did not believed Putin himself directly ordered the killing.
The incident sparked a years-long flurry of accusations and investigations.
In January 2007, British officials said police had solved the killing after discovering "a 'hot' teapot at London's Millennium Hotel with an off-the-charts reading for polonium-210.
The UK's 2016 public inquiry found that Putin probably approved a Russian intelligence operation to murder Litvinenko, carried out by two state agents, Andrei Lugovoi and Dmitry Kovtun, directed by Russia's Federal Security Service (FSB), the main successor to the Soviet-era KGB.
Then-Prime Minister David Cameron subsequently accused Russia of 'state-sponsored' murder, which the Kremin denied.
British demands to Moscow that the Kremlin cooperate with the UK investigation, including by extraditing Lugovoi and Kovtun, have long fallen flat.
Today, the ECHR agreed Litvinenko's death was a Russian state-sponsored killing carried out by the pair, despite both men always denying involvement.
"The court found it established, beyond reasonable doubt, that the assassination had been carried out by Mr Lugovoy and Mr Kovtun," the ruling said.
The court also included that had the duo been carrying out a "rogue operation", as some theories suggested, Moscow would have had the information to prove it.
"However, the government had made no serious attempt to provide such information or to counter the findings of the UK authorities," the ruling said.
"The planned and complex operation involving the procurement of a rare deadly poison, the travel arrangements for the pair, and repeated and sustained attempts to administer the poison indicated that Mr Litvinenko had been the target of the operation."
The 2018 Salisbury spy poisoning saga resurrected fears of Russian agents active on British soil.
Sergei Skripal and daughter Yulia survived the Novochok nerve poisoning attempt on their lives.
Two passersby who came upon the poison vessel, a perfume bottle, also fell ill.
Dawn Sturgess died on 8 July, but Charlie Rowley, who also came into contact with the poison, and a police officer who attended the incident, both survived.
Putin has denied involvement in the Skripal case, and the Kremlin has openly mocked British attempts to engage Russia with the investigation.
Two men claiming to be the suspects named by the UK appeared on Russian state television said they had been merely been tourists visiting Salisbury to observe its cathedral's "famous ... 123-metre spire".
