Mr Litvinenko was a former Russian FSB agent who went on to work with MI6 after fleeing to the UK.
The 43-year-old, an outspoken critic of Russian President Vladimir Putin, died in 2006 after drinking green tea laced with the rare and very potent radioactive isotope Polonium 210 at London’s Millennium Hotel.
“Russia was responsible for assassination of Aleksandr Litvinenko in the UK,” the court said in a statement on its ruling.
A UK 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. Both men have always denied involvement.
“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 on Tuesday.
"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 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."
It too concluded that the Russian state was to blame and that had the men been carrying out a "rogue operation", Moscow would have the information to prove that theory.
"However, the government had made no serious attempt to provide such information or to counter the findings of the UK authorities," the ruling said.