The murder of ex-Russian spy Alexander Litvinenko in 2006 in the UK was probably approved by President Vladimir Putin, a public inquiry has found.
Putin is likely to have signed off the poisoning of Litvinenko with polonium-210 in part due to personal "antagonism" between the pair, it said.
Home Secretary Theresa May said his murder was a "blatant and unacceptable" breach of international law.
But the Russian Foreign Ministry said the report had been "politicised".
It said: "We regret that the purely criminal case was politicised and overshadowed the general atmosphere of bilateral relations."
Litvinenko's widow Marina welcomed the report, calling for sanctions to be imposed on Russia and a travel ban on Putin.
Litvinenko died aged 43 in London in 2006, days after drinking tea poisoned with the radioactive substance. The former Russian spy had fled to Britain where he became a fierce critic of the Kremlin.
The long-awaited report into his death found two Russian men - Andrei Lugovoi and Dmitry Kovtun - deliberately poisoned Litvinenko. They both deny killing him.
Sir Robert Owen, the inquiry chairman, said Lugovoi and Kovtun were probably acting under the direction of Moscow's FSB intelligence service.
Giving a statement to the House of Commons, May said the UK would now impose asset freezes on Lugovoi and Kovtun.
International arrest warrants against the pair also remained in place, she told MPs.
Prime Minister David Cameron would also raise the findings with President Putin at "the next available opportunity", May added.
A Downing Street spokeswoman said the report's conclusions were "extremely disturbing", saying: "It is not the way for any state, let alone a permanent member of the UN Security Council, to behave."
Speaking earlier outside London's High Court, Litvinenko said: "The words my husband spoke on his deathbed when he accused Putin have been proved by an English court."
She urged the UK government to expel all Russian intelligence operatives and impose economic sanctions on Moscow.
The conclusions of this inquiry are stronger than many expected in pointing the finger at Vladimir Putin personally.
The evidence behind that seems to have come from secret intelligence heard in closed session.
Saying that Alexander Litvinenko was killed because he was an enemy of the Russian state will raise pressure on the British government to take real action - the steps taken nearly a decade ago were only limited in scope.
That may pose difficulties given the importance of Russia's role in the Middle East, but without tough action people may ask if the Russian government has been allowed to get away with what has been described as an act of nuclear terrorism on the streets of London.
Responding to the report, Lugovoi, who is now a politician in Russia, said the accusations against him were "absurd", the Russian news agency Interfax was quoted as saying.
"As we expected, there were no surprises," he said.
"The results of the investigation made public today yet again confirm London's anti-Russian position, its blinkeredness and the unwillingness of the English to establish the true reason of Litvinenko's death."
Kovtun, now a businessman in Russia, said he would not comment on the report until he got more information about its contents, Interfax reported.
London's Metropolitan Police said the investigation into the "cold and calculated murder" remained ongoing.
Publishing his report, Sir Robert said he was "sure" Litvinenko's murder had been carried out by Lugovoy and Kovtun.
Both are wanted in the UK for questioning, but Russia has refused to extradite them.
Singling out then-FSB chief Nikolai Patrushev, alongside Putin, he said: "Taking full account of all the evidence and analysis available to me I find that the FSB operation to kill Litvinenko was probably approved by Patrushev and also by President Putin."
Sir Robert said Litvinenko's work for British intelligence agencies, his criticism of the FSB and Putin, and his association with other Russian dissidents were possible motives for his killing.
There was "undoubtedly a personal dimension to the antagonism" between Putin and Litvinenko, he added.
"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 Litvinenko, including killing him, in late 2006," Sir Robert wrote.
The use of polonium 210 was "at the very least a strong indicator of state involvement" as it had to be made in a nuclear reactor, the report said.
The inquiry heard evidence that Litvinenko may have been consigned to a slow death from radiation to "send a message".