May will chair a meeting of the UK’s National Security Council and then is expected to make a statement to Parliament that could have a profound effect on UK-Russia relations.
She had demanded that Moscow respond by midnight Tuesday, London time, to the UK government’s conclusion that Russia was linked to the poisoning of Sergei Skripal and his daughter Yulia on a park bench in Salisbury.
The Skripals are critically ill in hospital after being exposed to the nerve agent, known as Novichok and developed in Russia, on March 4. Thirty-eight other people in Salisbury were seen by medics after the exposure. One, a police officer, remains in hospital.
May said Monday it was highly likely that Moscow was behind the poisoning. The Russian ambassador to the United Kingdom was summoned to the UK Foreign Office to explain whether the attack was directed by Russian authorities, or whether Moscow had lost control of the nerve agent.
Russian Foreign Minister Sergey Lavrov dismissed accusations of Russian involvement in the attack as “nonsense,” state-run Tass reported. His office said Moscow would not respond to London’s ultimatum without receiving a sample of the substance, setting up a potential showdown between Downing Street and the Kremlin.
Skripal is believed to have lived in the UK since his release from Russian custody in 2010.
He was convicted in Russia of spying for Britain before he was granted asylum in the UK after a high-profile spy swap in 2010 between the United States and Russia.
Since then, a number of Russians have been attacked or died in the United Kingdom. The latest is Nikolai Glushkov, a Russian exile who was found dead in his London home on Monday night.
Glushkov had links to compatriots who died in unexplained circumstances in the UK, but police said there was no evidence to suggest a link to Glushkov’s death and Skripal’s poisoning.
British Foreign Minister Boris Johnson said the attack against the Skripals likely the first time a nerve agent has been used in Europe since the end of World War II.
Novichok is believed to be up to 10 times as potent as VX, the nerve agent used to kill Kim Jong Nam, the estranged older half-brother of North Korean leader Kim Jong Un, at an airport in Kuala Lumpur last year.
France, Germany and the United States have all condemned the attack. US President Donald Trump said he believes that the evidence, as it stands, points to Russia as the responsible party.
“It sounds to me like they believe it was Russia and I would certainly take that finding as fact,” Trump said.
“As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be.”
CNN’s Judith Vonberg contributed to this report