It is "highly likely" that Russia was behind the nerve agent attack on Sergei Skripal and his daughter Yulia, British prime minister Theresa May had told the UK Parliament.
May said the substance used was a "military grade" nerve agent produced by Russia and there were only two possible explanations - either Moscow was behind the attack or it had lost control of its stockpile of the poison.
The Prime Minister said Russia's ambassador Alexander Yakovenko had been summoned to the British Foreign Office to explain what happened.
Foreign Secretary Boris Johnson has told him that Moscow must "immediately provide full and complete disclosure" of its novichok nerve gas programme to the Organisation for the Prohibition of Chemical Weapons, giving Russia until the end of Tuesday to respond, said May.
In a dramatic statement after a meeting of the National Security Council, during which she received the latest intelligence analysis and an update on the investigation, May told MPs: "It is now clear that Mr Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia."
The Kremlin has denied the involvement of the Russian government in the nerve agent attack on the Skripals.
Following May's statement, news agency Tass quoted Russian foreign ministry spokeswoman Maria Zakharova as saying: "It is a circus show in the British parliament.
"The conclusion is obvious, it's another political information campaign, based on a provocation."
But May said: "On Wednesday we will consider in detail the response from the Russian State.
"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom."
The National Security Council is expected to meet on Wednesday to discuss the Russian response, if any, ahead of a statement by the PM.
Ahead of the Prime Minister's statement, the Russian embassy accused the UK Government of playing a "very dangerous game" with British public opinion and warned of the risk of "serious long-term consequences."