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 PM said the government concluded it was "highly likely" Russia was responsible for the Salisbury attack.
Russia's ambassador had been asked to explain whether it was "direct action" by the state or due to it "losing control" of its nerve agent stock.
She said the UK must "stand ready to take much more extensive measures".
The chemical used in the attack, the PM said, has been identified as being part of a group of nerve agents known as "Novichok".
The name Novichok means "newcomer" in Russian, and applies to a group of advanced nerve agents developed in secret by the Soviet Union in the 1970s and 1980s.
One of the chemicals - called A-230 - is reportedly five to eight times more toxic than VX nerve agent, which can kill a person within minutes.
Mrs May said Foreign Secretary Boris Johnson had told the ambassador Moscow must provide "full and complete disclosure" of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons by the end of Tuesday.
She said the UK would then consider their 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.
Detective 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.
Russia said on Monday that Ms May's allegations were politically motivated and based on a provocation.
"It is a circus show in the British parliament," the TASS news agency quoted Russian Foreign Ministry spokeswoman Maria Zakharova as saying.
"The conclusion is obvious: It's another political information campaign, based on a provocation."
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.
- BBC, Reuters