Russia has strongly rejected accusations that it was behind a second poisoning in Britain involving the deadly nerve agent Novichok.
A British couple remain in a critical condition in hospital after coming into contact with it on Saturday.
A Kremlin spokesman said on Thursday the allegation were baseless.
"The British side has not presented any evidence of Russia's involvement in this, besides unfounded accusations. You know that the Russian side has offered to conduct a joint probe with the British from the very beginning, but this offer was left without any answer," Dmitry Peskov said.
There's been much speculation about the timing of the incident as Russia hosts the World Cup and one MP from President Putin's Russia United party echoed allegations already disseminated in the Russian media.
"This is happening during a quite strong positiv e emotional wave from British fans who have come to Russia to support their team during the World Cup. This of course goes against what the British authorities and the British media are saying about Russia," Sergei Zhelezn Yak said.
Much of Russia's media have said the poisoning was a cynical anti-Russian ploy.
And Russia's foreign ministry accused the British government of being behind the attack.
"For the sake of security of the citizens of our continent we call the government of Theresa May to stop intrigues and games with poisonous agents," Maria Zakharova, a Russian foreign ministry spokeswoman, said.
The British security services are trying to establish whether the novichok was left over from the attack on former Russian spy Sergei Skripal and his daughter.