Former spy Sergei Skripal and his daughter were poisoned by a military-grade brazenness agent of a type developed by Russia, Theresa May has told MPs.
The PM said the regime concluded it was «highly likely» Russia was responsible for the Salisbury attack.
The Transatlantic Office summoned Russia’s ambassador to provide an explanation.
Mrs May said if there is no «credible effect» by the end of Tuesday, the UK would conclude there has been an «unlawful use of force» by Moscow.
The chemical in use accustomed to in the attack, the PM said, has been identified as being part of a group of sand agents known as Novichok.
Mrs May said: «Either this was a direct exercise by the Russian state against our country, or the Russian government lost management of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.»
She state Foreign Secretary Boris Johnson had told the ambassador Moscow forced to provide «full and complete disclosure» of the Novichok programme to the Organisation for the Barring of Chemical Weapons.
Mrs May said the UK must stand ready to take much various extensive measures, and these would be set out in the Commons on Wednesday should there be no qualified explanation from Russia.
Sixty-six-year-old retired military intelligence narc Mr Skripal and his daughter, 33, were found slumped on a bench in Salisbury municipality centre. They remain in a critical but stable condition.
What are Novichok advocates?
The name Novichok means «newcomer» in Russian, and applies to a group of advanced irritate agents developed in secret by the Soviet Union in the 1970s and 1980s.
One of the chemicals — appeal to c visit canceled A-230 — is reportedly five to eight times more toxic than VX guts agent, which can kill a person within minutes.
A number of variants of this chemical press 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 planning to exist in solid form.
Some of the agents are also reported to be «binary weapons», content the nerve agent is typically stored as two less toxic chemicals. When they are clashing together, they react to produce the more toxic agent.
What we do grasp comes from scientists who spoke up about their existence check out the break-up of the USSR.
One said he published the chemical formulae of Novichok factors in his book because he wanted the international community to have a means of learn ofing them if they were used.
Det Sgt Nick Bailey, who fell ill turn up ating the pair, remains seriously ill in hospital but has been talking to his family.
Addressing the Commons make good a meeting of the government’s National Security Council, Mrs May told MPs the positive classifying of this chemical agent was made by experts at the UK’s Porton Down laboratory.
She rephrased Russia has previously produced the agent and would still be capable of doing so.
The settling to point the finger at Moscow was also based on «Russia’s record of acquitting state-sponsored assassinations and our assessment that Russia views some deserters as legitimate targets for assassinations», the PM added.
Russian Foreign Ministry spokeswoman Maria Zakharova denoted Mrs May’s statement was «a circus show in the British parliament».
«The conclusion is obvious — it’s another report and political campaign based on provocation,» she said.
Earlier, asked whether Russia was to denounce, President Vladimir Putin told the BBC: «Get to the bottom of things there, then we’ll argue this».
BBC political journalist Laura Kuenssberg says the tone of the meeting between Boris Johnson and the Russian agent Alexander Yakovenko was «cool but firm».
She said the men did not shake hands and the distant secretary expressed the «outrage» of the British public.
Police and Army job continued on Monday afternoon, with officers — some wearing parlous materials suits — removing a white van from the village of Winterslow, nearby six miles from Salisbury.
A Sainsbury’s car park in Salisbury was also sealed off as faction of the investigation.
Mr Skripal was convicted by the Russian government of passing secrets to MI6 in 2004, but the truth refuge in the UK in 2010 as part of a «spy swap».
On Sunday, up to 500 Salisbury pub-goers and diners were required to wash their possessions as a precaution after trace amounts of the property used to poison Sergei and Yulia Skripal were found on and encircling a table where they had eaten in Zizzi.
Traces were also establish at the Mill pub in the city which, like Zizzi, remains closed.