The Kremlin says British accusations that Russian President Vladimir Putin was involved in a nerve agent attack in England are shocking and unforgivable, TASS news agency reports.
British Foreign Secretary Boris Johnson said earlier on Friday that it was overwhelmingly likely that Putin himself made the decision to strike down a former Russian agent on English soil.
"Our quarrel is with Putin's Kremlin and with his decision," said Mr Johnson, "and we think it is overwhelmingly likely that it was his decision to direct the use of a nerve agent on the streets of the UK for the first time since the Second World War."
"Any reference or mention of our president in this regard is a shocking and unforgivable breach of diplomatic rules of decent behaviour," Kremlin spokesman Dmitry Peskov said on Friday, according to the agency.
Russia has denied any involvement, cast Britain as a post-colonial power unsettled by Brexit, and even suggested London fabricated the attack in an attempt to whip up anti-Russian hysteria.
Asked by a Reuters reporter in the Kazakh capital if Russia planned to expel British diplomats from Moscow, Russian Foreign Minister Sergei Lavrov smiled and said: "We will, of course."
Britain, the United States, Germany and France jointly called on Russia on Thursday to explain the attack. US President Donald Trump said it looked as though the Russians were behind it.
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Russia has refused Britain's demands to explain how Novichok, a nerve agent developed by the Soviet military, was used to strike down Sergei Skripal, 66, and his daughter Yulia, 33, in the southern English city of Salisbury.
Mr Skripal, a former colonel in the GRU who betrayed dozens of Russian agents to British intelligence, and his daughter have been critically ill since March 4, when they were found unconscious on a bench.
A British policeman who was also poisoned when he went to help them is in a serious but stable condition.
President Vladimir Putin, a former KGB spy who is poised to win a fourth term in an election on Sunday, has so far only said publicly that Britain should get to the bottom of what has happened.
In a sign of just how tense the relationship has become, British and Russian ministers used openly insulting language while the Russian ambassador said London was trying to divert attention from the difficulties it was having managing Britain's exit from the European Union.
Defence Secretary Gavin Williamson sparked particular outrage in Moscow with his blunt comment on Thursday that "Russia should go away, it should shut up."
Russia's Defence Ministry said he was an "intellectual impotent" and Mr Lavrov said he probably lacked education. Mr Williamson studied social science at the University of Bradford.
"Well he's a nice man, I'm told, maybe he wants to claim a place in history by making some bold statements," Mr Lavrov said.
"Theresa May's main argument about Russia's guilt is 'highly probable', while for him it's 'Russia should go and shut up'. Maybe he lacks education, I don't know."
In London, opposition Labour Party leader Jeremy Corbyn struck a starkly different tone to that of the British government by warning against rushing into a new Cold War before full evidence of Moscow's culpability was proven.
"To rush way ahead of the evidence being gathered by the police, in a fevered parliamentary atmosphere, serves neither justice nor our national security," the 68-year-old socialist leader wrote in the Guardian newspaper.
Mr Corbyn said Labour did not support Mr Putin and that Russia should be held to account if it was behind the attack.