The murder of ex-Russian spy Alexander Litvinenko in 2006 in the UK was “probably” approved by President Vladimir Putin, an inquiry has found.
Mr Putin is likely to have signed off the poisoning of Mr Litvinenko with polonium-210 in part due to personal “antagonism” between the pair, it said.
Home Secretary Theresa May said the murder was a “blatant and unacceptable” breach of international law.
But the Russian Foreign Ministry said the public inquiry was “politicised”.
It said: “We regret that the purely criminal case was politicised and overshadowed the general atmosphere of bilateral relations.”
The long-awaited report into Mr Litvinenko’s death found that two Russian men – Andrei Lugovoi and Dmitry Kovtun – deliberately poisoned the 43-year-old in London in 2006 by putting the radioactive substance polonium-210 into his drink at a hotel.
Sir Robert Owen, the public inquiry chairman, said he was “sure” Mr Litvinenko’s murder had been carried out by the two men and that they were probably acting under the direction of Moscow’s FSB intelligence service, and approved by the organisation’s chief, Nikolai Patrushev, as well as the Russian president.
He said Mr Litvinenko’s work for British intelligence agencies, his criticism of the FSB and Mr Putin, and his association with other Russian dissidents were possible motives for his killing.
There was also “undoubtedly a personal dimension to the antagonism” between Mr Putin and Mr Litvinenko, he said.
‘Send a message’
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 Mr Litvinenko may have been consigned to a slow death from radiation to “send a message”.
Giving a statement to the House of Commons, Mrs May said Prime Minister David Cameron would raise the findings with President Putin at “the next available opportunity”.
She said the UK would now impose asset freezes on Mr Lugovoi and Mr Kovtun and that international arrest warrants for the pair remained in place. They both deny killing Mr Litvinenko.
Both men are wanted in the UK for questioning, but Russia has refused to extradite them.
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.”
Shadow home secretary Andy Burnham described the murder of Mr Litvinenko as an “act of state-sanctioned terrorism” and “an attack on London, sanctioned at the very highest levels of the Russian government”.
“Given that, I don’t believe the government’s response today went anywhere near far enough,” he said.
Speaking earlier outside the High Court, Mr Litvinenko’s widow, Marina, said she was “very happy” that “the words my husband spoke on his deathbed when he accused Mr Putin have been proved by an English court”.
She urged the UK government to expel all Russian intelligence operatives, impose economic sanctions on Moscow and impose a travel ban on Mr Putin.
Responding to the report, Mr 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.”
Mr 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.
Alexander Yakovenko, the Russian ambassador in the UK, said Russia would not accept any decisions reached in secret and based on evidence not tested in open court.
The length of time taken to come to these conclusions led them to believe it was “a whitewash of British security services’ incompetence”, he said.
Mr Yakovenko said these events “can’t help but harm our bilateral relations”.
Mr Litvinenko fled to the UK in 2000, claiming persecution. He was granted asylum and gained British citizenship several years later.
In the years before his death, he worked as a writer and journalist, becoming a strong critic of the Kremlin.
It is believed he also worked as a consultant for MI6, specialising in Russian organised crime.
The inquiry heard from 62 witnesses in six months of hearings and was shown secret intelligence evidence about Mr Litvinenko and his links with British intelligence agencies.