Former Russian spy Sergei Skripal, 66 and his daughter Yulia, 33 were found collapsed after being poisoned on Sunday afternoon.
Skripal and Yulia remain in a serious condition along with a police officer who came in contact with the same substance.
May today chaired a meeting of the National Security Council (NSC) to assess the intelligence around the poisoning, BBC reported.
"And we will not tolerate such a brazen attempt to murder innocent civilians on our soil," she added.
The prime minister said the government had concluded it was "highly likely" that Russia was responsible for the attack on Skripal and his daughter Yulia in Salisbury on March 4.
She said Russia's ambassador in London had been summoned to explain whether it was "a direct action by the Russian state" or the result of it "losing control" of its stock of nerve agents.
Colonel Skripal was convicted of treason in 2006 and jailed for 13 years for selling secrets to MI6, which had recruited him in the 1990s. The senior intelligence officer with Russian military intelligence GRU, was pardoned in a spy swap in 2010 and settled in Salisbury, Wiltshire.
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