President Vladimir Putin probably approved a Russian intelligence operation to murder ex-KBG agent Alexander Litvinenko, a judge led-British inquiry into the 2006 killing in London concluded.
Litvinenko, 43, an outspoken critic of Putin who fled Russia, died after drinking green tea laced with the rare radioactive isotope polonium-210 at London's plush Millennium Hotel.
An inquiry led by senior judge Robert Owen found that former KGB bodyguard Andrei Lugovoy and fellow Russian Dmitry Kovtun carried out the poisoning as part of an operation directed by Russia's Federal Security Service (FSB), the main heir to the Soviet-era KGB.
"Taking full account of all the evidence and analysis available to me, I find that the FSB operation to kill Mr Litvinenko was probably approved by Mr Patrushev and also by President Putin," the inquiry said.
Nikolai Patrushev was formerly head of the FSB.
"I am satisfied that in general terms members of the Putin administration, including the President himself and the FSB, had motives for taking action against Mr Litvinenko, including killing him in late 2006," the inquiry said.
The Kremlin has always denied any involvement. From his deathbed, Litvinenko told detectives Putin had directly ordered his killing.
The death of Litvinenko marked a post-Cold War low point in Anglo-Russian relations, and ties have never recovered, marred further by Russia's annexation of Crimea and its support for Syrian President Bashar al-Assad.
Both Lugovoy and Kovtun have previously denied involvement and Russia has refused to extradite them. Lugovoy was quoted by the Interfax news agency as saying the accusation was absurd.
The widow of Alexander Litvinenko has called for targeted sanctions against Vladimir Putin, saying it would be unthinkable for UK Prime Minister David Cameron not to take action after an inquiry found the Russian President probably approved the murder of her husband on British soil.
Marina Litvinenko said she was "very pleased" at the result of the inquiry, which found strong evidence suggesting Litvinenko was poisoned to death on the orders of Russia's intelligence agency, the FSB, in an operation approved by its then chief, Nikolai Patrushev, and the Kremlin.
After the inquiry report into the death of her husband was published, Litvinenko addressed Cameron: "I am calling immediately for the expulsion from the UK of all Russian intelligence operatives, whether from the FSB or other Russian agencies based in the London Embassy."
"I am also calling for the imposition of targeted economic sanctions and travel bans against named individuals, including Mr Patrushev and Mr Putin. It is unthinkable that the prime minister would do nothing in the face of the damning findings of [inquiry chairman] Sir Robert Owen," said Marina Litvinenko.
Alexander Litvinenko, a former Russian secret service agent who had received asylum in the UK, died after being poisoned with Polonium-210 in London in November 2006.
Marina Litvinenko's barrister Ben Emmerson told a press conference that due to the high radioactivity of the substance used, public health and security were also put at risk by the killers. "There were more than 100 individuals who were potentially contaminated and 200 sites in London where contamination traces were found. This is an issue of public health; an issue of danger in the streets.
"It would be surprising if the prime minister, who prides himself with keeping the streets of London safe from terrorism, would sit on his hands in the face of judicial findings not just of nuclear terrorism but of state-sponsored nuclear terrorism," said Emmerson.
The inquiry concluded that the two main suspects in the case, Andrei Lugovoi and Dmitry Kovtun, "probably" poisoned Litvinenko by lacing his tea at a the Millennium Hotel in Mayfair. The pair weren't present during the six-month hearings as Russia has refused to extradite them. Both men deny any involvement in murder.
According to the report, in the years following Litvinenko's death in 2006, Putin actively "supported and protected" Lugovoi, and also handed him an honour for services to the fatherland while the inquiry was taking place.
Marina Litvinenko said she was positive the killers of her husband will one day pay for their crimes. "Yes, they are walking the streets of Moscow today but I believe there will be a final day when these two people will be punished," she said.
The case has far-reaching diplomatic implications, with Russia playing a key role both in the crisis in Ukraine and Syria. Many analysts are doubtful the British government will take strong measures against Putin.
After the report was published swift action has been taken instead against Lugovoi and Kovtun. Interpol notices and European arrest warrants have been issued for the two suspects and the British Treasury has confirmed an asset freeze targeting the pair, while as the government considers further action.