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.
He confirmed Russia's receipt on Thursday of a classified report from the Organization for the Prohibition of Chemical Weapons, which the organization sent after confirming Britain's identification of a toxic agent used in the poisoning.
MI6 double agent Sergei Skripal and his daughter Yulia are believed to have been poisoned at his Salisbury home by chemicals daubed on his front door.
"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent global weapons controls".
"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", he said.
In his letter, Sedwill outlines how the Soviet Union "developed a new class of "fourth generation" nerve agents, known as Novichoks" during the 1980s and Russian Federation was probably the only former Soviet republic to pursue "an offensive chemical weapons programme after independence".
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".
'The Russian state has previously produced Novichoks and would still be capable of doing so'.
"If someone was spying, why were the British services not complaining about that?"
He said that Russian Federation had continued to produce and stockpile small quantities of Novichoks within the last decade. A United Kingdom public inquiry concluded there was a "strong probability" the poisoning was directed by the FSB, Russia's top security agency.
Britain blamed Russian Federation for the poisoning and Prime Minister Theresa May said the Skripals had been attacked with a military-grade nerve agent from the Novichok group of poisons, developed by the Soviet Union.
Calling on North Atlantic Treaty Organisation to take action, he concluded: "I know that North Atlantic Treaty Organisation will remain seized of the need to confront the increasingly aggressive pattern of Russian Federation behaviour of which the attack in Salisbury was an acute and recent example".
"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.
Skripal, 66, and his daughter, 33, were admitted to a hospital after being found unconscious on a public bench on March 4. Sergei Kripal remains hospitalised, but British health officials say he is recovering.
"I want to underline: the OPCW only has confirmed the composition of the chemical agent", he said.