The Russian Foreign Ministry said British special services could have been behind the poisoning of a former Russian spy in Salisbury.
Britain has accused Russia of a nerve agent attack on former double agent Sergei Skripal and his daughter Yulia.
Russia has fiercely rejected the accusations, calling them baseless.
More than 20 nations have expelled over 130 Russian diplomats this week in a show of solidarity with Britain.
Russia denounced the actions as "boorish" and pledged to retaliate.
The Russian Foreign ministry's claim of "possible involvement" of British special services in the poisoning marks Moscow's bluntest claim yet of alleged British foul play.
British officials have previously rejected such Russian claims as nonsense.
Moscow said the British failure to provide evidence suggests the poisoning was a "major political provocation".