"It is highly likely that Russia was responsible for the act against Sergei and (his daughter) Yulia Skripal," May said, telling British lawmakers that the military-grade nerve agent used in the attack was of a type developed by Russia.
"Either this was a direct act by the Russian state against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others," the prime minister added.
May stopped short of announcing retaliatory measures against Moscow and instead gave the Kremlin until the end of Tuesday to disclose details of the Novichok nerve agents programme to the Organisation for the Prohibition of Chemical Weapons.
The organisation is the implementing body of the Chemical Weapons Convention, which entered into force in 1997 and has 192 member states.
The nerve agent used in the March 4 attack in the southwestern city of Salisbury was identified by experts at a British army base, while May said Russia had a record of "state-sponsored assassinations".
"This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals," she told lawmakers.
"It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk. And we will not tolerate such a brazen attempt to murder innocent civilians on our soil," May concluded.
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