The UK appears to be pursuing a policy of "destroying evidence" in the case of the poisoning of former double agent Sergei Skripal, the Russian envoy to the UK said during a press conference on Friday. "We didn't see any signs, any applications from the British side that they are not happy with the way Skripals were living in Salisbury".
Moscow has hit back by expelling Western diplomats, questioning how Britain knows that Russian Federation was responsible and suggesting it stemmed from a plot by British secret services.
Scientists from the Organization for the Prohibition of Chemical Weapons (OPWC) confirmed Thursday that Novichok was used in Salisbury, England in the attack on Skripal.
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".
"The code word used for the offensive chemical weapons programme (of which the Novichoks were one part) was FOLIANT", he said.
Richard Guthrie, an independent chemical-weapons expert, says an important detail in the investigation is that the toxic substance is of "high purity".
He said that after the Soviet Union collapsed, Russian Federation signed the Chemical Weapons Convention without reporting its ongoing work on Novichoks.
He then describes how Russian Federation later launched a program to test methods of delivering chemical warfare agents and to train personnel in their use.
He pointed to the Owen Report from the UK's public inquiry into the death of Aleksandr Litvinenko which concluded in 2016 that he was "deliberately poisoned with Polonium 210, that there was a "strong probability" that the FSB (Russia's security agency) directed the operation, and that President Putin "probably approved it".
Among the techniques they had investigated for delivering nerve agents was applying them to door handles, he said.
The letter adds that Russian intelligence services "view at least some of its defectors as legitimate targets for assassination".
Mr Sedwill wrote: "We have information indicating Russian intelligence service interest in the Skripals, dating back at least as far as 2013, when email accounts belonging to Yulia Skripal were targeted by GRU [Russian military intelligence] cyber specialists". "Within the last decade, Russian Federation has produced and stockpiled small quantities of Novichoks under the same programme".
He also questioned the authenticity of a statement issued by Ms Skripal through the Metropolitan Police saying she did not want to meet Russian officials.
Mr Yakovenko complained at the continued refusal of the British authorities to grant consular access to Ms Skripal following her discharge from hospital.
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".
Speaking on Friday, Sergey Lavrov, Russia's foreign minister, said the OPCW report offers nothing to support Britain's claims that Russia was behind the attack.
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.