Sergei Skripal, a former colonel in Russian military intelligence who betrayed dozens of agents to Britain's MI6 foreign spy service, and his daughter were found unconscious on a bench in the English cathedral city of Salisbury on March 4.
Sedwill also said that Russian Federation has tested means of delivering chemical agents "including by application to door handles", pointing out that the highest concentration of the chemical found after the attack was on Skripal's front door handle.
The group, the Organization for the Prohibition of Chemical Weapons, released a report saying that its laboratory analysis of "environmental and biomedical samples" that its experts had collected "confirm the findings of United Kingdom relating to the identity of the toxic chemical".
Britain named it as Novichok, a group of powerful and deadly chemical compounds reportedly developed by the Soviet government in the 1970s and 1980s.
The finding, announced by the Organization for the Prohibition of Chemical Weapons (OPCW), an global chemical weapons watchdog, was immediately jumped on by British Foreign Secretary Boris Johnson as an indictment of Russia's culpability in the attack.
Sedwill's letter to NATO Secretary General Jens Stoltenberg alleged that Russia had "the technical means, operational experience and motive for the attack on the Skripals and that it is highly likely that the Russian state was responsible".
'We would like to make sure that the statement really belongs to Yulia, ' the embassy said.
However, Georgy Kalamanov, Russia's deputy minister of industry and trade, said it was impossible to pinpoint the nerve agent's origin and reiterated Moscow's demand for a fresh investigation with Russian involvement.
Russian Federation claimed to have destroyed its chemical weapons stockpiles but the letter says it has "produced and stockpiled" small quantities of Novichok "within the last decade".
Salisbury District Hospital's medical director, Dr Christine Blanshard, said that Ms Skripal's being discharged was not the end of her treatment, but marked a "significant milestone".
Russian President Vladimir Putin, who served as a KGB operative between 1975 and 1991, said Moscow "does not possess such agents" and dismissed claims of Russia being behind the attack as "nonsense".
"With no possibility to verify it, the publication by the Metropolitan Police raises new questions rather than gives answers".
She added that her father remains "seriously ill", and that she is suffering from the attack.
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