Britain has accused Russia of being responsible for the use of a Soviet-era nerve agent called Novichok which was used to poison Sergei Skripal and his daughter Yulia.
The Kremlin said on Monday that London would either have to back up its assertions that Russia was behind the poisoning of a former spy and his daughter in Britain with evidence or apologise "sooner or later."
Britain has accused Russia of being responsible for the use of a Soviet-era nerve agent called Novichok which was used to poison Sergei Skripal and his daughter Yulia.
Kremlin spokesman Dmitry Peskov told a conference call with reporters that Britain's allegations were "difficult to explain... groundless and slanderous".
Russian President Vladimir Putin said on Sunday it was nonsense to think that Moscow was responsible for the attack.
British Foreign Secretary Boris Johnson said on Monday that Russia's denials were "increasingly absurd."
British Prime Minister Theresa May said it was "highly likely" Moscow was to blame after Britain identified the substance as part of the Novichok group of nerve agents which were developed by the Soviet military during the 1970s and 1980s.
"It is now clear that Mr Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia," May said.
"Either this was a direct act by the Russian state against our country. Or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others."
Russia said May's allegations were politically motivated.
May said Russia had shown a pattern of aggression including the annexation of Crimea and the murder of former KGB agent Alexander Litvinenko, who died in 2006 after drinking green tea laced with radioactive polonium-210.
A British public inquiry found the killing of Litvinenko had probably been approved by Putin and carried out by two Russians, Dmitry Kovtun and Andrei Lugovoy - a former KGB bodyguard who later became a member of the Russian parliament.
Both denied responsibility, as did Moscow.
Putin, who took over as Kremlin chief from Boris Yeltsin on the last day of 1999, has denied allegations that Russia meddled in the 2016 U.S. presidential election and says the West has repeatedly tried to undermine Russian interests.
Skripal betrayed dozens of Russian agents to British intelligence before his arrest in Moscow in 2004. He was sentenced to 13 years in prison in 2006, and in 2010 was given refuge in Britain after being exchanged for Russian spies.
Since emerging from the John le Carre world of high espionage and betrayal, Skripal lived modestly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday.
Novichok agents are believed to be five to 10 times more lethal than the more commonly known VX and Sarin. They cause a slowing of the heart and restriction of the airways, leading to death by asphyxiation, University of Reading pharmacology professor Gary Stephens said.
Russian state television said Skripal had been recruited by the British when working as Russia's military attache in Spain and that he had handed over 20,000 pages of secret documents to MI6, Britain's foreign spy service.
A British policeman who was one of the first to attend to the stricken spy was also affected by the nerve agent. He is now conscious in a serious but stable condition, police said.
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