He also said Russia had a clear motive for attacking Skripal, a former Russian military intelligence officer who had been imprisoned in Russia for spying for Britain only to be set free in a spy swap.
By 1993, when Russia signed the Chemical Weapons Convention, Sedwill said it was "likely" that some Novichoks had passed testing to allow their use by the Russian military.
'The Russian state has previously produced Novichoks and would still be capable of doing so'.
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.
The Skripals were targeted by a nerve agent attack that left both of them critically ill for weeks.
The letter adds that Russian intelligence services "view at least some of its defectors as legitimate targets for assassination".
"The codeword for the offensive chemical weapons programme (of which Novichoks were one part) was FOLIANT", he said.
"This program subsequently included investigation of ways of delivering nerve agents, including by application to door handles", it says.
Oscar-winning director Milos Forman passes away at 86
Larry Flint." " Man on the Moon ", based on the life of cult hero Andy Kaufman, did win its star, Jim Carrey, a Golden Globe. Three years later while Forman was in bed with a fever, the Gestapo intruded again, this time at the family home.
"We have information indicating Russian intelligence service interest in the Skripals, dating back at least as far as 2013, when e-mail accounts belong to Yulia were targeted by GRU cyber specialists", Sedwill wrote. The poisoning happened in the city of Salisbury shortly after she arrived from Moscow for a visit.
The Russian Embassy has sensationally claimed poisoned former spy Sergei Skripal and his daughter Yulia are being held prisoner by the UK. Sergei Kripal remains hospitalised, but British health officials say he is recovering.
The letter comes as British officials try to consolidate European support for united actions against Russian Federation.
Russia's Foreign Minister Sergey Lavrov said Friday that a new report on the poisoning of a former spy does nothing to support Britain's contention that Russian Federation was behind the attack. It demanded access to her and prodded Britain to share evidence in the case.
The note demanded the withdrawal of one Georgian diplomat now serving in Moscow at the Swiss Embassy Interests Section, as Georgia and Russia have no diplomatic ties since the Russia-Georgia 2008 war and Switzerland has acted as a mediator.
"I want to underline: the OPCW only has confirmed the composition of the chemical agent", he said.
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.
Shortly after the attack in Salisbury, UK Home Secretary Amber Rudd asked police and intelligence agency MI5 to look into further allegations of Russian state involvement in a number of deaths in the UK, prompted by a Buzzfeed report alleging Russian links to 14 suspicious deaths in the UK.