May said she would be pushing for a “robust international response” when the UN Security Council meets later Wednesday in New York to discuss the attack on Sergei Skripal and his daughter on March 4.
Russia has repeatedly denied any involvement and its London embassy warned that May’s response was “totally unacceptable and shortsighted.”
May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in the English city of Salisbury.
“There is no alternative conclusion other than that the Russian State was culpable for the attempted murder of Mr Skripal and his daughter,” she said.
“This represents an unlawful use of force by the Russian State against the United Kingdom.”
In measures drawn up at a meeting of her national security council earlier Wednesday, May announced that 23 Russian diplomats believed to be undeclared intelligence officers must leave Britain in a week.
She suspended all planned high-level contacts, which includes revoking an invitation for Foreign Minister Sergei Lavrov to visit but said she did not want to break off relations entirely.
May also confirmed that neither members of the royal family or ministers would attend the football World Cup in Russia later this year.
And she outlined fresh measures against people traveling to or living in Britain who were responsible for violations of human rights or planned “hostile activities.”
NATO allies, including the United States, have expressed their support for Britain following the first offensive use of a nerve agent in Europe since World War II.
Along with the UN Security Council meeting in New York, EU Council President Donald Tusk indicated that the issue would be on the agenda of next week’s summit of the bloc’s leaders in Brussels.
May said on Monday that it was “highly likely” that Russia was behind the attack, which left Skripal and his daughter in a critical condition in hospital, while a policeman was also hospitalized.
She had given Moscow until midnight Tuesday to explain whether it was directly responsible or “lost control” of the nerve agent, but said it has responded with “sarcasm, contempt and defiance.”
President Vladimir Putin’s spokesman said again Wednesday that it had “nothing to do with the accident in Britain,” but warned it would not accept the “language of ultimatums.”
Lavrov has said the Kremlin is ready to cooperate with Britain but complained that its request for samples of the nerve agent had been rejected.
Moscow has also warned that it will take retaliatory measures, and on Tuesday threatened to expel British media from Russia if the license of its state broadcaster RT was threatened in Britain.
May on Wednesday blamed Putin for a deterioration of relations between Moscow and London, saying it was “tragic that President Putin has chosen to act in this way.”
But the Russian embassy said the British government was responsible.
Britain is wary of acting alone and May has spoken to US President Donald Trump, French President Emmanuel Macron and German Chancellor Angela Merkel in recent days.
In a phone call late Tuesday, Trump and May “agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms,” the White House said.
In a joint statement by its 29 member states, the US-led NATO alliance said the attack was a “clear breach of international norms and agreements” and called on Russia to fully disclose details of the Novichok program.
British experts say Skripal, 66, and his 33-year-old daughter, who was visiting from Russia, were poisoned with a nerve agent from a broad category known as Novichok, which was developed by the Soviet Union during the late stages of the Cold War.
The Russian chemist who first revealed the existence of Novichok, Vil Mirzayanov, said “only the Russians” developed the Novichok agents.
“They kept it and are still keeping it in secrecy,” he said from his home in the US, where he moved in 1995 after 30 years of working for the State Scientific Research Institute of Organic Chemistry and Technology.
The Salisbury case has drawn parallels with the 2006 death by radiation poisoning of former Russian agent and Kremlin critic Alexander Litvinenko, which Britain blamed on Moscow.
In a further twist, former senior Russian executive Nikolai Glushkov, linked to late Kremlin opponent Boris Berezovsky, was found dead in London on Tuesday in unexplained circumstances, British and Russian media reported.