- 22 updates
The Litvinenko inquiry will "seriously complicate Russian-British ties", Russian Foreign Minister Sergei Lavrov said.
British government leaders could be sued for slander for their comments over the Litvinenko inquiry, Lavrov told a news conference.
The judge-led inquiry found President Vladimir Putin "probably" approved the assassination of former Russian spy Alexander Litvinenko in London.
Sir Robert Owen's report said Andrei Lugovoi and Dmitri Kovtun were probably acting under the direction of Moscow's FSB intelligence service when they poisoned him with radioactive polonium 210 at the Millennium Hotel in Mayfair in 2006.
He added he inquiry contained groundless accusations and left many questions unanswered.
Former Russian spy Andrei Lugovoi has dismissed as "nonsense" the inquiry that painted him and Dmitri Kovtun as one of Alexander Litvinenko's state-sponsored assassins.
Speaking to the BBC, the ex-KGB bodyguard and FSB agent dismissed the findings of the inquiry that said President Vladimir Putin "probably" signed off the murder he was tasked with executing.
He said: "I've seen the nonsense conclusions of your judge who has clearly gone mad.
"I saw nothing new there. I am very sorry that 10 years on nothing new has been presented, only invention, supposition, rumours."
Both men fled back to Russia and did not return to Britain to give evidence at the year-long inquiry, which returned its findings on Thursday.
Sir Robert Owen's report following a public inquiry into the murder of Alexander Litvinenko has now caused a major diplomatic row between Russia and the UK.
Mr Litvinenko's widow, Marina, is calling on Prime Minister David Cameron to take action against President Vladimir Putin after the report concluded that he "probably" approved the assassination.
However Russia says the report is a white-wash.
ITV News Deputy Political Editor Chris Ship spoke to Mr Litvinenko's widow after the report was published:
The British government has not ruled out "taking further steps" against suspects in the Alexander Litvinenko case, in addition to asset freezes already in place.
Speaking after an inquiry into Litvinenko's death published its final report a Downing Street official said the death was an "unacceptable breach of international law" and suggested further action may be taken against those involved in the death of the former Russian spy.
The office of the Prime Minister also noted that the findings of the report, showed Russia had behaved in a manner that was "not a way for a state to behave, especially not a permanent member of the United Nations security council".
Prime Minister David Cameron has said the findings of a report into the death of former spy Alexander Litvinenko has confirmed the Russian government's involvement - a belief that the UK government has had for some time.
Speaking in Davos, Cameron said "what happened was absolutely appalling" and Britain would be "toughening" up its response to Russia after the inquiry implicated Vladimir Putin in the assassination.
He added that the UK's relationship with Russia needed to continue in order to find a solution to the Syria crisis but said all future relations would be entered into with "clear eyes and a very cold heart".
An image of the teapot used to poison Russian dissident Alexander Litvinenko have been released.
Traces of polonium 210 were discovered in the teapot in the Pine Bar at the Millennium Hotel on November 1, 2006.
Sir Robert Owen's inquiry concluded Andrei Lugovoi and Dmitri Kovtun placed the polonium 210 in the teapot with the intention of poisoning Mr Litvinenko.
The Russia state's likely involvement in the murder of Alexander Litvinenko is "deeply disturbing, demonstrating a flagrant disregard for UK law" and "the safety of UK citizens", Europe minister David Lidington has told the Russian ambassador.
In a meeting at the Foreign Office, Mr Lidington told Alexander Vladimirovich Yakovenko of the Government's "deep concern" about the findings and warned it would complicate relations and "undermine trust" between the two nations.
“Mr Lidington reiterated the UK Government’s demand that Russia cooperate with the criminal investigation to ensure that the suspects could be brought to justice and face trial in the United Kingdom," a Foreign Office spokeswoman said.
Speaking outside the Foreign Office, Mr Yakovenko described the report as a "whitewash" and said it was "absolutely unacceptable" for the report to conclude the Russian state was involved in Mr Litvinenko's death.
The possible "state sponsored" murder of Litvinenko on the streets of London is "something we in this country simply cannot ignore", Boris Johnson has told ITV News.
The Mayor of London said if it is true that Putin or agencies of the Russian state were behind the murder then it "deserves the fiercest possible diplomatic response".
Latest ITV News reports
Some of the key details of the Alexander Litvinenko case broken down into numbers.
Sir Robert Owen's report into the death of Alexander Litvinenko identified a number of reasons why he could have been killed.