Retired High Court Judge Robert Owen, who conducted the inquiry, wrote that he was "sure" that two former Russian agents poisoned the 44-year-old at a London hotel with highly radioactive polonium-210.
And Owen wrote that he was also sure that the two men who allegedly poisoned Litvinenko -- former KGB and FSB employee Andrei Lugovoi and former Russian army officer Dmitri Kovtun -- were acting on behalf of others, probably the Russian spy service, the FSB.
"The FSB operation to kill Mr. Litvinenko was probably approved by Mr. Patrushev and also by President Putin," Owen wrote. Nikolai Patrushev was head of the FSB in 2006.
In response, the Russian Foreign Ministry dismissed the UK inquiry as politically motivated. "We regret that a purely criminal case has been politicized and has darkened the general atmosphere of our bilateral relations," spokeswoman Maria Zakharova said Thursday.
UK Home Secretary Theresa May told Parliament on Thursday that Interpol notices and European arrest warrants were in place so that the main suspects would be arrested if they traveled abroad.
And at the request of Foreign Secretary Philip Hammond, Russian Ambassador Alexander Vladimirovich Yakovenko was summoned to the UK Foreign Office to meet with David Lidington, the minister responsible for Russia.
"The Minister set out the UK Government's deep concern regarding the findings of the independent Litvinenko Inquiry report," a Foreign Office spokeswoman said. "He made clear that the inquiry's conclusion concerning the Russian State's probable involvement in this murder was deeply disturbing, demonstrating a flagrant disregard for UK law, international law and standards of conduct, and the safety of UK citizens. The Minister said that this would further complicate bilateral relations, undermine trust, and damage Russia's reputation internationally."
Lidington stressed that it was unacceptable for Russia to fail to honor a formal request to extradite Lugovoi.
The findings related to Putin -- even if his involvement is viewed as probable rather than definitively proved -- will offer some comfort to Litvinenko's widow, Marina. She said outside the High Court on Thursday she was "pleased that the words my husband spoke on his deathbed when he accused Mr. Putin of his murder have been proved true in an English court."
In an interview later in the day with CNN, she accused Putin of direct involvement.
"Putin himself personally protected Lugovoi. He himself," she said. "He provided his immunity. ... He protected him. He grant him to be a member of parliament. It means he took all this responsibility."
Who was Alexander Litvinenko?
Litvinenko was a former Russian security agent who came to Britain in 2000 after turning whistleblower on the FSB, the successor agency to the KGB, the former Soviet secret police and intelligence agency. (FSB stands for its name in Russian, Federalnaya Sluzhba Bezopastnosti, which translates as Federal Security Service
According to his wife, he then started working for Britain's security services.
Litvinenko had many enemies. His last job at the FSB was heading up its anti-corruption department. It was in the chaos of the 1990s, after the collapse of the Soviet Union, which may have brought him into conflict with ruthless colleagues.
In Owen's report, he quotes a Russian saying, "There is no such thing as a former KGB man."
After leaving the FSB, Litvinenko blamed the service for orchestrating a series of apartment bombings in Russia in 1999 that left hundreds dead and led to Russia's invasion of Chechnya
later that year.
All or any of the above may have been sufficient reason to kill a man seen by his former FSB colleagues as a traitor -- a man who once even published an article, cited in Owen's report, that accused Putin of being a pedophile.
What caused his death?
In a deathbed statement, read to the media by a friend, Litvinenko blamed Putin for ordering his poisoning, saying the rare radioactive substance polonium-210 was slipped into his tea at a meeting with the two suspects in Mayfair in 2006.
He said: "You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life."
The Kremlin has always denied the accusation, as did Lugovoi and Kovtun, whom the Russian government refuses to extradite to Britain. But the Kremlin's case isn't helped by the compelling evidence against it.
Polonium-210 occurs naturally, but scientists say the high concentrations found in the Litvinenko case indicate production at a nuclear reactor, or perhaps a particle accelerator -- the kinds of facilities, in other words, controlled by a state.
Russia isn't the only country capable of producing polonium-210. But the radioactive trail left by the substance was traced across London, while three British Airways aircraft that had flown a number of routes, including London to Moscow, were reported to be contaminated with an unnamed radioactive substance.
Whom does the family hold responsible?
Litvinenko's family blames Putin, whom Litvinenko criticized repeatedly.
Marina Litvinenko and her legal team pointed the finger at Putin repeatedly in statements to the inquiry, which was set up in February 2014 by May, the home secretary.
"My husband was killed by agents of the Russian state in the first-ever act of nuclear terrorism on the streets of London," Marina Litvinenko said. "This could not have happened without knowledge or consent of Mr. Putin."
On Thursday, she added: "Now it is time for (British Prime Minister) David Cameron
. I am calling immediately for the expulsion from the UK of all Russian intelligence operatives, whether from the FSB or for other Russian agencies based in the London Embassy.
"I'm also calling for the imposing of targeted economic sanctions and travel bans against named individuals, including Mr. Putin."