Russia has branded Theresa May’s suggestion that Moscow was probably behind the Salisbury poisonings as a “circus show”.
Updating MPs, the Prime Minister said it was “highly likely” that Russia was responsible for what happened.
The poison used to target former spy Sergei Scripal and his daughter Yulia was from the Russian-made nerve agent group novichok, the PM said.
But Moscow hit back, claiming Mrs May’s statement was “another political information campaign based on a provocation”.
Russian Foreign Ministry spokeswoman Maria Zakharova said there had been a “circus show in the British parliament”.
And Moscow added, cryptically: “Before making up new fairy tales, let the British disclose how the Litvinenko case ended.”
Russian dissident Alexander Litvinenko was poisoned with radioactive polonium-210 in London in 2006.
His widow, Marina, says Britain appears unable to protect those seeking political asylum in the UK.
Sergei Scripal, 66, and Yulia, 33, have been in a critical condition in hospital since being found unconscious in Salisbury on Sunday 4 March.
Mrs May said their poisonings took place “against a backdrop of a well-established pattern of Russian state aggression”.
Britain was ready to take “much more extensive measures” against Russia than in the past, she added.