The 43-year-old, who had worked for the Russian security services before defecting to the UK, died after drinking green tea laced with poison in London in 2006.
The European Court of Human Rights ruled on Tuesday that Russia was behind his assassination, which was carried out at the Millennium Hotel in Mayfair.
Russia has always denied any involvement in Litvinenko’s death.
A British inquiry concluded in 2016 that Vladimir Putin probably approved a Russian intelligence operation to murder Litvinenko, who was an outspoken critic of the Russian president.
It found ex-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).
On Tuesday, the European Court of Human Rights said it had found there was “a strong prima facie case that both men “had been acting as agents of the Russian State” in poisoning Litvinenko.
It said: “The Court found that there was no evidence that either man had had any personal reason to kill Mr Litvinenko and that, if acting on their own behalf, they would not have had access to the rare radioactive isotope used to poison him.”
Litvinenko, who had worked for both the FSB and KGB, went public with allegations in 1998 that he had been asked to look into the possibility of killing a wealthy businessman.
He fled Russia after being fired by the FSB and was granted asylum in the UK in 2001.
In its ruling, the European Court of Human Rights said Litvinenko “became involved in exposing corruption and links to organised crime in the Russian intelligence services”.
The former spy was poisoned with a rare and very potent radioactive isotope called Polonium 210 in London in 2006. He died around three weeks after falling ill with symptoms including vomiting and abdominal pain.
Additional reporting by Reuters
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