According to the Foreign Office, the UK has information that Russia has produced and stored a small amount of the poison gas.
A British Government spokesperson said Porton Down's identification of Novichok was "only one part of the intelligence picture".
"This includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination — and as part of this programme has produced and stockpiled small quantities of Novichok; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets. As the Prime Minister has set out in a number of statements to the Commons since 12 March, this includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination — and as part of this programme has produced and stockpiled small quantities of Novichoks," he noted.
London also took into consideration "Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets."
"It is our assessment that Russia was responsible for this brazen and reckless act and, as the international community agrees, there is no other plausible explanation," the Foreign Office statement read.
Earlier in the day, Sky News reported that Porton Down experts are unable to identify the precise source of the nerve agent that was used in poisoning the former Russian agent, but that it was a military-grade nerve agent.
"We were able to identify it as Novichok, to identify that it was a military-grade nerve agent. We have not identified the precise source, but we have provided the scientific info to the government who have then used a number of other sources to piece together the conclusions you have come to," Gary Aitkenhead, chief executive of the Defence Science and Technology Laboratory (DSTL) at Porton Down stated.
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The UK side has declared that this substance is similar to the Novichok-class nerve agents developed in the Soviet Union.
UK Prime Minister Theresa May has blamed Russia for organizing the attack and expelled 23 Russian diplomats as a punitive measure. A number of countries have supported the UK claims and expelled more than 150 Russian diplomats.
The Russian side has denied all the accusations and suggested participating jointly in the investigation. However, Moscow's request for samples was ignored. Moscow in turn also expelled Western countries diplomats and ordered the British Council to stop its activities in Russia in response to London's move.