Updating MPs on the attack the Prime Minister said it is now clear they were poisoned by a military grade nerve agent of a type developed by the Russian state.
It comes after Ms May was briefed in Downing Street by police and intelligence chiefs on the latest developments in the case at a meeting of the National Security Council (NSC).
The Kremlin has denied involvement of the Russian government in the nerve agent attack on the Skripals but in an extraordinary statement, Ms May told MPs gathered in the Commons that Russia has previously produced this nerve agent and “would still be capable of doing so”.
She continued: “Russia’s record of conducting state sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassination, the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.”
The Prime Minister said the poison belonged to a group of nerve agents known as “novichok”, adding: “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 to get it into the hands of others.”
Ms May also described last week’s attack as an “indiscriminate” and “reckless” act against the United Kingdom that put the lives of innocent civilians at risk.
“And we will not tolerate such a brazen attempt to murder innocent civilians on our soil,” she added.
Responding to the Prime Minister’s statement on the attack, Jeremy Corbyn faced shouts of “shame” and “disgrace” from Conservative MPs as he told the Commons: “We’re all familiar with the way huge fortunes, often acquired in the most dubious circumstances in Russia, sometimes connected with criminal elements, have ended up sheltering in London and trying to buy political influence in British party politics.
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“Meddling in elections, as the Prime Minister put it, and there has been over £800,000 worth of donations to the Conservative Party from Russian oligarchs and their associates.”
The Labour leader added: “We need to continue seeking a robust dialogue with Russia on all the issues dividing our countries, both domestic and international – rather than simply cutting off contact and simply letting tensions and divisions get worse, and potentially even more dangerous.”