A former Russian spy and his daughter were poisoned by a military-grade nerve agent of a type developed by Russia, Theresa May has told MPs.
The prime minister said the government had concluded it was "highly likely" that Russia was responsible for the attack on Sergei Skripal and his daughter Yulia in Salisbury on 4 March.
She said Russia's ambassador in London had been summoned to explain whether it was "a direct action by the Russian state" or the result of it "losing control" of its stock of nerve agents.
The chemical used in the attack, the PM said, has been identified as being part of a group of nerve agents known as "Novichok".
Mrs May said Foreign Secretary Boris Johnson had told the ambassador Moscow must "immediately provide full and complete disclosure" of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons.
She said the UK would consider his response before deciding what action to take, but added: "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."
Sixty-six-year-old retired military intelligence officer Sergei Skripal and his daughter, 33, were found slumped on a bench in Salisbury city centre. They remain in a critical but stable condition.
Det Sgt Nick Bailey, who fell ill attending the pair, remains seriously ill in hospital but has been talking to his family.
Addressing the Commons following a meeting of the Government's National Security Council, Mrs May told MPs the positive identification of this chemical agent was made by experts at the UK's Porton Down laboratory.
She said Russia has previously produced the agent and would still be capable of doing so.
The decision to point the finger at Moscow was based on "Russia's record of conducting state-sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassinations", the PM added.
The BBC reports that police activity continued on Monday afternoon, with officers - some wearing hazardous materials suits- removing a white van from the village of Winterslow, about six miles from Salisbury.
A Sainsbury's car park in Salisbury was also sealed off by police.
