Mr Sedwill - who published the letter on Twitter on Friday - 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.
Email accounts of Yulia had been targeted in 2013 by cyber specialists from Russia's GRU military intelligence service, Mark Sedwill said in a letter to NATO Secretary General Jens Stoltenberg on Friday.
He said, based on British intelligence, that Russian Federation had developed a next generation nerve-agent group called Novichok and stockpiled small amounts of the agent within the last decade. The Russian side flatly rejected all of the United Kingdom's accusations, saying that a program aimed at developing such a substance had existed neither in the Soviet Union nor in Russia.
'The Russian state has previously produced Novichoks and would still be capable of doing so'.
In the letter, Sedwill said the Soviet Union developed fourth generation nerve agents known as Novichoks in the 1980s at the State Institute for Organic Chemistry and Technology near Volgograd under the codeword FOLIANT. It insists that the nerve agent used on the Skripals could easily have been manufactured in any of the other countries that have advanced chemical research programs.
Her father remains hospitalized but is no longer in critical condition.
Sir Mark said Russian Federation had a "proven record of conducting state-sponsored assassination" and that it was "highly likely" some defectors may be regarded as "legitimate targets".
Last week, the chief executive of the Defense Science and Technology Laboratory in Porton Down, Britain's premier chemical weapons laboratory, said its scientists could not identify "the precise source" of the chemical, though its purity indicated that it was nearly certainly created by a "state actor".
Mr Sedwill added Moscow 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.
Sony PlayStation 5 unlikely to launch before 2020
Instead of figuring out when the PlayStation 5 will be launched, gamers should instead enjoy what the PlayStation 4 offers. The game will also ask players to make "tough, sometimes hilarious" decisions either in single-player or two-player co-op.
British royals welcome new baby
The new baby will become fifth in line to the throne of England upon its arrival, bumping uncle Prince Harry to sixth in line. Middleton and William are already parents to son Prince George and daughter Princess Charlotte .
Cynthia Nixon joined Eric B and Rakim on stage in NY
She also criticized the lack of "political courage coming out of Albany" for not having already legalizing marijuana in NY . Rob Stone shared footage of the concert on Instagram , with the caption, "When Cynthia Nixon shows up at Eric B ".
Sedwill's letter to the North Atlantic Treaty Organisation chief detailed the grounds on which British authorities hold Russian Federation responsible of a suspected nerve agent attack targeting Skripal and his daughter Yulia in Salisbury six weeks ago.
Ms Zakharova, Russia's Foreign Ministry spokeswoman, challenged Britain to prove the Skripals were not being held hostage, noting that no-one except British authorities had seen either of them for over a month.
Also Friday, Russian Foreign Minister Sergey Lavrov responded to an global chemical weapons watchdog's report on the poisoning.
Lavrov said the report does nothing to back the British allegations that Moscow was behind the attack.
Separately on Friday, Russia's ambassador to Britain said he was concerned the British government was trying to get rid of evidence related to the case.
"I want to underline: the OPCW only has confirmed the composition of the chemical agent", Lavrov said at a news conference.
He briefed journalists on the latest development in Douma, a town in the suburbs of Damascus, where the alleged attack took place and where Russian military police has been deployed on Thursday.
Finally, on the motive, Sedwill said Sergei Skripal was a former Russian military intelligence (GRU) officer, convicted of espionage in 2004.