Former spy Sergei Skripal 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.
The Foreign Office has asked Russia’s ambassador for an explanation.
Mrs May said if there is no “credible response” by the end of Tuesday, the UK would conclude there has been an “unlawful use of force” by Moscow.
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: “Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.”
She 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.
Mrs May said the UK must stand ready to take much more extensive measures, and these would be set out in the Commons on Wednesday should there be no adequate explanation from Russia.
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.
A number of variants of this chemical have been manufactured, and one of them was reportedly approved for use by the Russian military as a chemical weapon.
While some are liquids, others are thought to exist in solid form.
Some of the agents are also reported to be “binary weapons”, meaning the nerve agent is typically stored as two less toxic chemicals. When they are mixed together, they react to produce the more toxic agent.
What we do know comes from scientists who spoke up about their existence following the break-up of the USSR.
One said he published the chemical formulae of Novichok agents in his book because he wanted the international community to have a means of detecting them if they were used.BBCNEWS