"Mr. Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russian Federation", specifically from the Novichok group, British Prime Minister Theresa May said after the incident, based on an assessment by British experts.
"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 (military intelligence) cyber specialists", he said.
As it was stated in the letter, Russian special services were conducting experiments with the Novichok nerve agent for years, they put the nerve agent on the door-handle, in particular.
Both Mr. Skripal and his daughter were found unconscious on a bench in the town of Salisbury in early March, after they were poisoned.
"This program subsequently included investigation of ways of delivering nerve agents, including by application to door handles", it says. The claims come after the global Organisation for the Prohibition of Chemical Weapons backed Britain's assertion the Skripals were poisoned by Novichok - a military grade nerve agent developed by the Russians in the 1980s. "There is no plausible alternative explanation".
Alexander Yakovenko, Russia's ambassador to the United Kingdom, dismissed Sedwill's charges as unfounded and untrue.
The Foreign Ministry says that it is Russia's response to Georgia's previous decision over the Skripal case.
Yulia Skripal tells Russia to leave her alone after being poisoned with nerve agent
"It is unlikely that Novichok could be made and deployed by non-state actors", he adds. "Within the last decade, Russian Federation has produced and stockpiled small quantities of Novichoks under the same program".
"The codeword for the offensive chemical weapons programme (of which Novichoks were one part) was FOLIANT", he said.
"It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassinations", Sedwill writes, alluding to a possible motive for the attack on Sergei Skripal, a former Russian military official convicted of being a British spy.
Mr Yakovenko complained at the continued refusal of the British authorities to grant consular access to Ms Skripal following her discharge from hospital.
"We are not allowed to see our citizens, talk to doctors, have no idea about the treatment the Russian nationals receive", he said.
On Wednesday she said she did not now wish to take up an offer of consular assistance from the Russian embassy, according to a note released by London's Metropolitan Police.
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