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."
Dmitry Peskov, Mr Putin's spokesman, said Moscow's official response to the report will happen through "diplomatic channels", the Russian news agency Interfax was quoted as saying.
Prime Minister David Cameron said the UK would have to go on having "some sort of relationship with them [Russia]" because of the Syria crisis, but it would be done with "clear eyes and a very cold heart".
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.