Moscow must explain why a Russian-made nerve agent was used in the poisoning of former Russian agent Sergei Skripal and his daughter by the end of Tuesday, Theresa May said.
The PM said it was “highly likely” Russia was responsible for the attack in Salisbury, Wiltshire, on 4 March.
Nato secretary-general Jens Stoltenberg said: “The UK is a highly valued ally, and this incident is of great concern to Nato.”
Russia called the claims “unfounded”.
US Secretary of State Rex Tillerson said it appeared the “really egregious act… clearly came from Russia” and there should be “serious consequences”.
Home Secretary Amber Rudd will chair a meeting of the government’s emergencies committee Cobra later to discuss the case.
Former double agent Mr Skripal, 66, and his daughter, Yulia, 33, were found slumped on a bench in Salisbury city centre. They remain in a critical but stable condition in hospital.
Det Sgt Nick Bailey, who fell ill attending to the pair, remains seriously ill, but has been talking to his family.
Mrs May told the Commons on Monday that the poison used in the attack was a military-grade nerve agent developed by Russia. She said it was part of a group of nerve agents known as Novichok.
“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.
Mrs May said the Foreign Office had summoned Russia’s ambassador to “explain which of these two possibilities it is”.
She warned that if there was no “credible response” by the end of Tuesday, the UK would conclude there has been an “unlawful use of force” by Moscow.
She said the UK must “stand ready to take much more extensive measures” against Russia than it had previously.
She said these measures would be set out in the Commons on Wednesday should there be no adequate explanation from Russia.
She described the poisoning as “an indiscriminate and reckless act against the UK, putting the lives of innocent civilians at risk”.
Conservative minister Dominic Raab said sanctions could be wide-ranging but played down the involvement of Nato.
He told Radio 4’s Today programme that Mrs May “chose her words very carefully” in her Commons statement on the incident.
‘Really egregious act’
Mr Tillerson, who spoke to Foreign Secretary Boris Johnson on the phone about the case on Monday, said the US supported the UK’s assessment that Russia was likely responsible.
He added: “We agree that those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences.
“We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses.”
He described it as a “really egregious act” and said he had become “extremely concerned” by Russia becoming “more aggressive”.
Mrs May also spoke to French President Emmanuel Macron on Monday and the two leaders “agreed that it would be important to continue to act in concert with allies” to address what it called “the wide pattern of aggressive Russian behaviour”, her spokesman said.
Former foreign secretary David Miliband said the PM should focus on seeking international support over the next two days.
He told Radio 4’s Today programme: “I think that rallying the European allies, and, if possible, significant strands of American opinion is absolutely key.”
Russian foreign ministry spokeswoman Maria Zakharova said Mrs May’s statement was “a circus show in the British parliament”.
“The conclusion is obvious – it’s another information and political campaign based on provocation,” she said.
Yevgeny Serebrennikov, first deputy chair of the Russian Federation Council’s Defence and Security Committee, described Mrs May’s accusations as “unfounded”.
Earlier, asked whether Russia was to blame, President Vladimir Putin told the BBC: “Get to the bottom of things there, then we’ll discuss this.”
What are Novichok agents?
- The name 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 chemical – called A-230 – is reportedly five to eight times more toxic than VX nerve agent, which can kill a person within minutes
- Some are liquids, others are thought to exist in solid form. Some are reported to be “binary weapons”, meaning they are typically stored as two less toxic chemicals which when mixed, react to produce the more toxic agent
- One variant was reportedly approved for use by the Russian military as a chemical weapon
- Designed to escape detection by international inspectors, their existence was revealed by defectors