Russia has dismissed the findings of a British inquiry into the death of Russian dissident Alexander Litvinenko as a possible "joke".
The report by Sir Robert Owen found that Russian President Vladimir Putin "probably" signed off the poisoning of the dissident with radioactive polonium in London in 2006.
A spokesperson for Mr Putin said it is likely the findings "can be attributed to fine British humour".
Dmitry Peskov said: "The fact that an open public inquiry is based on the classified data of special services, unnamed special services, and that the verdict which has been made on the basis of this flimsy data has been made public with the copious use of the words 'probably' and 'likely'."
He also said that such a "quasi-inquiry" could only "add more poison to the atmosphere of our bilateral ties".
British Prime Minister David Cameron has insisted Britain was "toughening" up its response to Russia in the wake of the report.
However, Mr Cameron conceded the UK must continue to have "some sort" of relationship with Russia.
Mr Cameron said the UK would deal with Russia with "clear eyes and a very cold heart".
Speaking in Davos, Mr Cameron described the state-sponsored murder as "absolutely appalling" but said the inquiry report confirms what "we have always believed" had happened.
Mr Owen's 300-page report said Andrei Lugovoi and Dmitry Kovtun were probably acting under the direction of Moscow's FSB intelligence service when they poisoned the 43-year-old with radioactive polonium 210 at the Millennium Hotel in Mayfair.
Singling out then-FSB chief Nikolai Patrushev alongside Mr Putin, Sir Robert wrote: "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 Mr Patrushev and also by President Putin."
Following the conclusion of the inquiry's verdict Mr Litvinenko's widow Marina Litvinenko, called for "targeted economic sanctions" against Russia and a travel ban on President Vladimir Putin.
During the inquiry, Mr Litvinenko's death was described as "a nuclear attack on the streets of London".
Traces of the polonium which killed him were found at numerous locations including offices, hotels and planes.
Mr Litvinenko ingested the poison while drinking tea after meeting two former KGB colleagues at the hotel.
He had been an outspoken critic of Mr Putin and from his death bed accused the president of involvement in the poisoning.
It was a claim rejected by the Kremlin as politically motivated.
If today's findings suggest there was high level Russian involvement in the death, it would further sour what are already difficult diplomatic relations between the UK and Russia.
Russian ambassador dismisses inquiry as 'whitewash'
Earlier, the Russian Ambassador to London called the inquiry a "whitewash".
Speaking outside the foreign office after he was summoned to a meeting by Europe minister David Lidington, Alexander Yakovenko said the findings represented a "gross provocation" by Britain.
He said: "The length of time that it took to close this case makes us believe it to be a whitewash of the British special services' incompetence."
He described at as "absolutely unacceptable" for the report to conclude the Russian state was involved in Mr Litvinenko's death.
He added: "This gross provocation of the British authorities cannot help hurting our bilateral relations."
His meeting at the Foreign Office lasted less than an hour.
Mr Lidington told Mr Yakovenko "the Russian State's probable involvement in this murder was deeply disturbing, demonstrating a flagrant disregard for UK law, international law and standards of conduct, and the safety of UK citizens", a spokeswoman for the foreign office said.
White teapot centre of investigation
A white teapot was at the centre of the murder inquiry that shocked the world and plunged Britain's relations with Russia into crisis.
The inquiry into Mr Litvinenko's death concluded that Lugovoi and Kovtun placed polonium 210 in a teapot at the hotel's Pine Bar.
Every teapot at the hotel was tested and only one bore evidence of contamination, the probe heard.
Extremely high readings were detected on the porcelain pot, which was given the exhibit number NJH/1. The highest levels were found on the inside of the spout.
One expert estimated that at least 50 micrograms of polonium 210 was placed into the teapot.
The inquiry report said forensic evidence showed the Pine Bar was heavily contaminated, with the highest readings taken from the table where Mr Litvinenko was sitting and from the inside of one of the teapots.
"No comparable levels of contamination were found in any of the other places that Mr Litvinenko visited that day," it said.
In his conclusions, inquiry chairman Sir Robert Owen wrote: "I am sure that Mr Lugovoi and Mr Kovtun placed the polonium 210 in the teapot at the Pine Bar on November 1 2006."