It is "highly likely" that Russia was behind the nerve agent attack on Sergei Skripal and his daughter Yulia, Theresa May told has MPs.
The Prime Minister 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 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 Mrs 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, Mrs 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."
She added: "Based on the positive identification of this chemical agent by world-leading experts at the Defence Science and Technology Laboratory at Porton Down; our knowledge that Russia has previously produced this agent and would still be capable of doing so; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations; the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal."
That meant "either this was a direct act by the Russian state against our country" or Vladimir Putin's government had "lost control of this potentially catastrophically damaging nerve agent".
The Kremlin has denied the involvement of the Russian government in the nerve agent attack on the Skripals.
But Mrs 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."
That would result in Mrs May setting out "the full range of measures that we will take in response".
She said: "This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals.
"It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk.
"And we will not tolerate such a brazen attempt to murder innocent civilians on our soil. "
Mrs May used her statement to pay tribute to Wiltshire Police Detective Sergeant Nick Bailey, who is in a serious but stable condition in the Salisbury District Hospital.
Commons Foreign Affairs Committee chairman Tom Tugendhat said Russia's actions were "deeply threatening to the British people".
Meanwhile investigators wearing hazardous materials suits have been working in the village of Winterslow about six miles from Salisbury.
Police and Army teams were at a site where a white van was loaded onto a truck ready to be taken away.
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".
In a statement on the embassy website, a spokesman said: "We would like to stress once again that we are outraged by the anti-Russian media campaign, condoned by the Government, that influences the investigation and has a psychological effect on British residents."
Russian President Vladimir Putin dismissed questions about his state's alleged involvement in the Skripal case.
On a visit to a grain centre, he told the BBC: "We're dealing with agriculture here ... and you talk to me about some tragedies.
"Get to the bottom of things there, then we'll discuss this."