Russia was responsible for the 2006 killing of a former KGB officer in London.
That was the ruling made by the European Court of Human Rights on Tuesday (September 21).
Alexander Litvinenko died three weeks after drinking green tea laced with polonium-210 at London's Millennium hotel.
Images of him lying yellow and gaunt in a hospital bed with his hair falling out were seen in media across the world.
Britain has long blamed Moscow for the attack, which sent relations to a post Cold War low.
Russia has always denied any involvement in the death of the then 43 year-old Kremlin critic.
A previous British inquiry had concluded in 2016 that Russian President Vladimir Putin probably approved an intelligence operation to murder Litvinenko.
It also found that former KGB bodyguard Andrei Lugovoy and another Russian Dmitry Kovtun carried out the killing - and that Russia's Federal Security Service - or FSB - probably directed the operation.
Five years on from that ruling and the ECHR agreed.
The court found that beyond reasonable doubt the assassination was carried out by both Lugovoy and Kovtun.
Both men have denied any involvement.
On Tuesday, Lugovoy told Reuters that the ruling was 'politically motivated' and 'extremely idiotic'.
"I want to highlight that I cannot call this ECHR decision other than improper and unlawful. Moreover, I'd like to call it stupid. It's not very clear on which foundation they based its decision. If they based it on what Litvinenko told them, it discredits ECHR reputation."
The ECHR also found that the Russian state was to blame.
It said if the men had been carrying out a "rogue operation" Moscow would have the information to prove that theory, but no such attempt was made to do so.