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 Russian Embassy has sensationally claimed poisoned former spy Sergei Skripal and his daughter Yulia are being held prisoner by the UK.
He added: "During the 2000s, Russian Federation commenced a programme to test means of delivering chemical warfare agents and to train personnel from special units in the use of these weapons".
Mark Sedwill also said in a letter to NATO Secretary General Jens Stoltenberg that Russian Federation had previously tested whether door handles could be used to deliver nerve agents.
The lethal poison that struck down a former Russian spy and his daughter in England last month was a highly pure type of Novichok nerve agent, the global chemical weapons watchdog has concluded, backing Britain's findings.
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".
"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent worldwide weapons controls".
National Security Security Adviser Sir Mark Seawall said Vladimir Putin's spies have trained to use nerve agents such as Novichok since the 2000s.
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Yulia Skripal, 33, has been released from the hospital.
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.
He then claimed Russian Federation saw at least some of its defectors as legitimate targets for assassination, suggesting Skripal could be among them.
It was "highly likely", he said, that some defectors - like Mr Skripal, a former GRU officer who was convicted of treason in 2006 and exchanged in a spy swap in 2010 - were regarded as "legitimate targets".
Regarding the Skripals themselves, Sedwill claims that email accounts belonging to Yulia Skripal were targeted by Russian intelligence agents in 2013, "indicating Russian intelligence service interest in the Skripals".
Russian Foreign Minister Sergei Lavrov noted on Friday that a report this week by the Organisation for the Prohibition of Chemical Weapons did not confirm the origin of the poison used against the Skripals. "Within the last decade, Russian Federation has produced and stockpiled small quantities of Novichoks under the same programme".
Mr Lavrov said the report only confirmed the composition of the substance and that Britain's claim that it confirmed the United Kingdom position on the Skripal case was overstated.