President Vladimir Putin has been linked to have approved the death of ex-Russian spy Alexander Litvinenko in 2006 in the UK, an investigation revealed.
According to BBC report 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 "politicized".
It said: "We regret that the purely criminal case was politicized 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 Inter fax 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.