Specialists investigating the incident have found the highest confrontation of the military-grade agent on the front door of Mr Sergei's Salisbury home.
In the letter, Sedwill said only Russian Federation had the "technical means, operational experience and the motive" for the attack.
Russian President Vladimir Putin, who served as a KGB operative between 1975 and 1991, said Moscow "does not possess such agents" and dismissed claims of Russia being behind the attack as "nonsense".
Britain named it as Novichok, a group of powerful and deadly chemical compounds developed by the Soviet government in the 1970s and 1980s.
U.K. Prime Minister Theresa May rallied allies in the U.S., Europe, Australia and Canada to conduct coordinated expulsions of more than 150 Russian diplomats last month after she told parliament it was "highly likely" that Russia was responsible.
Lavrov said neither Russian Federation nor the USSR developed BZ.
"The codeword for the offensive chemical weapons programme (of which Novichoks were one part) was FOLIANT", he said. 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.
The Skripals' poisoning provoked a diplomatic crisis between Russian Federation and the United Kingdom, as well as other Western governments, resulting in the expulsion of more than 150 diplomats on each side.
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British police officers in protective suits and masks at the scene in Salisbury where former double-agent Sergei Skripal and his daughter, Yulia were discovered after being attacked with a nerve-agent in March.
He said Russian Federation had a "proven record of conducting state-sponsored assassination" and that it was "highly likely" some defectors - like Mr Skripal, a former GRU officer who was exchanged in a spy swap in 2010 - may be regarded as "legitimate targets".
She has been interviewed by police and moved to a secure location after being released from hospital earlier this week.
The shocking news of a change in his condition followed the news a fortnight ago that the health of his daughter, Yulia, had also stabilised and that she was looking forward to leaving hospital.
Sir Mark, a senior official in the Cabinet Office, included the new detail - some of which had previously been leaked to British media - in a letter to NATO Secretary General Jens Stoltenberg briefing him on the poisoning of the Skripals in Salisbury. It demanded access to her and prodded Britain to share evidence in the case.
Russia's ambassador to Britain said a claim by a British security advisor on Friday that Russian Federation spied on former agent Sergei Skripal and his daughter for at least five years before they were attacked with a nerve agent was a "big surprise".
Lavrov said the report does nothing to back the British allegations that Moscow was behind the attack.
"I will underline that the OPCW confirmed only the contents of the chemical substance", he said.