British Prime Minister Theresa May said on Monday it was "highly likely" that Moscow was responsible for the poisoning in England of former Russian double agent Sergei Skripal and his daughter using a military-grade nerve agent.
Either the Russian state was directly responsible for the poisoning or it had allowed the poison, which belonged the Novichok group of nerve agents, to get into the hands of others, May told Britain's parliament.
Skripal, once a colonel in Russia's GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench in the southern English city of Salisbury last week.
The case is reminiscent of the 2006 murder of ex-KGB agent Alexander Litvinenko, who was killed with radioactive polonium-210 in London.
Skripal and his daughter remain in hospital in critical condition. A policeman who was one of the first to attend to the victims is in serious, but stable condition.
May said Russia's ambassador to London had been summoned to explain to British Foreign Secretary Boris Johnson how the nerve agent came to have been used in the March 4 incident.
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"On Wednesday we will consider in detail the response from the Russian state," May said.
"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," she said.
May said the poisoning took place "against a backdrop of a well-established pattern of Russian state aggression" and she said Britain was ready to take "much more extensive measures" against Russia than in the past.
Russia's foreign ministry hit back immediately, saying the comments by May were a "circus show" and part of political information campaign against Russia, local media reported.