The Foreign Ministry says that it is Russia's response to Georgia's previous decision over the Skripal case.
The UK's national security adviser said "credible open-source reporting and intelligence" showed that in the 1980s the Soviet Union developed a family of nerve agents known as Novichoks at a base in Shikhany near Volgograd.
Russia's ambassador to the UK Alexander Yakovenko described the claims as a "big surprise" and complained they had not been formally notified of the allegations by the UK authorities.
The letter reads: "I would like to share with you andallies further information regarding our assessment that it is highly likely that the Russian state was responsible for the Salisbury attack".
United Kingdom officials have said Sergei Skripal, 66, and his 33-year-old daughter, Yulia, were poisoned when they came in contact with a chemical nerve agent at his home in Salisbury on March 4.
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"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent global chemical weapons controls", Sedwill said in the letter.
Both Mr. Skripal and his daughter were found unconscious on a bench in the town of Salisbury in early March, after they were poisoned.
Claiming that the substance used in the incident had been a nerve agent allegedly developed in Russia, London rushed to accuse Moscow of being involved in the case without presenting any evidence.
He said cyber specialists from the GRU - Russian military intelligence - targeted Yulia's email accounts as far back as 2013.
He said Russian Federation had a proven record of state-sponsored assassinations and had tested ways of delivering chemical weapons, including the use of door handles to spread nerve agents, as Britain believes was done in the Skripal case.
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The report from the Organization for the Prohibition of Chemical Weapons did not include a statement about where the poison was manufactured.
Sedwill said within the last decade, Russian Federation has produced and stockpiled small quantities of Novichok, which was found on Sergei Skripal's front door.
"We get the impression the British Government is deliberately pursuing the policy of destroying all possible evidence".
Russian officials have strenuously denied any responsibility for the attack.
Russian Federation has continued to deny any involvement in the poisoning of the former spy and his daughter, asking for further investigation into what Sir Mark describes as an attack.
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Yulia was discharged from Salisbury District Hospital this week while her father remains in the hospital.