Theresa May, the British prime minister, has said the Skripals were attacked with one of the Novichok group of poisons, which had been developed by the Soviet Union in the 1970s and 80s.
At a press conference arranged before the letter's release, Russian ambassador to the U.K. Alexander Yakovenko said he was not familiar with the latest allegations. The same facility was named in a report by British newspaper The Times last week as the location where the nerve agent used on the Skripals was manufactured, with the paper citing British security sources.
He said only Russian Federation had the "technical means, operational experience and the motive" for the attack.
The poisoning of ex-GRU Colonel Sergei Skripal and his daughter Yulia in Salisbury, UK on March 4 was quickly blamed on Russian Federation by UK Prime Minister Theresa May, leading to a sharp deterioration in diplomatic ties.
Sedwill said that after the Soviet Union collapsed, Russian Federation signed the Chemical Weapons Convention without reporting its ongoing work on Novichoks.
"We therefore continue to judge that only Russia has 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", the letter says.
Russian Federation strongly denies the UK's claims about Novichok, saying it destroyed its entire Soviet-era chemical weapons arsenal a year ago under worldwide oversight.
Ecuador president says journalists were killed
The group is believed responsible for a string of recently deadly attacks in northern Ecuador against military targets. They were kidnapped on March 26 on the border of Colombia and Ecuador , according to the newspaper.
Risk for severe storms Friday
They said there also is a chance of an isolated tornado, although at this time that chance appears low. Temperatures will be falling through the 60s, and bottom out overnight in the middle and low 50.
Khloe Kardashian wants to leave Cleveland amid Tristan Thompson's cheating scandals
The video shows Tristan putting his face between one of the women's breasts before she gropes him. The athlete appears to kiss Marie, who is sitting on the left, and grabs her hand.
Sergei Skripal, 66, and his daughter Yulia, 33, were found unresponsive on a bench in Salisbury, southern England, on March 4.
Sedwill said Britain had information that Russian cyber specialists monitored Yulia's emails since at least 2013.
The report from the Organization for the Prohibition of Chemical Weapons did not include a statement about where the poison was manufactured.
He said that Russian Federation had continued to produce and stockpile small quantities of Novichoks within the last decade.
The central element of Britain's case against Russian Federation is the unusual nerve agent used in the attack, which was developed in Soviet laboratories during the last years of the Soviet Union.
"We get the impression that the British government is deliberately pursuing the policy of destroying all possible evidence, classifying all remaining materials and making an independent and transparent investigation impossible", Alexander Yakovenko told reporters in London.
He also complained at the continued refusal of the British authorities to grant consular access to Ms Skripal following her discharge from hospital. Her father remains in the hospital but British health officials say he is improving.