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 apologize "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."