Russia has angrily hit back after an official inquiry found the killing of dissident ex-spy Alexander Litvinenko was probably authorised by President Vladimir Putin.
Foreign minister Sergei Lavrov warned it would "certainly worsen" relations between London and Moscow and even suggested that British officials could be sued for slander.
The inquiry report published last week by Sir Robert Owen found Litvinenko died after ingesting radioactive polonium administered by two Russian agents in a London hotel in 2006 in an operation probably signed off by the Russian leader.
But at a news conference in Moscow, Mr Lavrov bitterly denounced the findings saying "grave accusations" had been made against the Russian leadership but there was "absolutely no proof" for the inquiry's conclusions.
"If an experienced lawyer takes up the case and analyses those facts and the statements made by top British government officials, there is enough material for a slander case," he said.
"All conclusions are based on the testimony of some specially selected witnesses, who are not objective ... or the testimony which has been classified, so it has remained unclear who have given it and whom it has been given to."
Mr Lavrov compared the report to the Dutch inquiry into the shooting down of Malaysian Airlines Flight MH17 over Ukraine – which found it was brought down by a Russian-made missile – and made clear it would have consequences for diplomatic relations.
"We heard what David Cameron and other Cabinet members have said. I particularly liked British Prime Minister saying he was shocked and that the inquiry has confirmed what they knew from the start," he said.
"It reminds me of what our American colleagues were saying after the crash of the Malaysian Boeing that they were waiting for a conclusive Dutch report even though we know already who had done it. It's obviously the same logical pattern in the Litvinenko case and in the Boeing case.
"I could only agree with the British Foreign Office saying that the Litvinenko case will further damage our relations. I fully agree with that, but it's not the Litvinenko's case, but a show around the Litvinenko case, that will hurt our relations. They will certainly worsen."
Following the report's publication, Home Secretary Theresa May said the findings were "deeply disturbing" and that Russian ambassador had been summoned to the Foreign Office to account for the Kremlin's role.
She also announced that the European arrest warrants had been issued for the two main suspects - Andrei Lugovoi and Dmitri Kovtun - and that asset freezes had been put in place against them by the Treasury.
The latest row comes at a difficult time for the Government as British diplomats are seeking to find ways of working with Moscow to end the Syrian civil war.