Britain has accused Russian intelligence agencies of spying on former double agent Sergei Skripal and his daughter for at least five years in the run up to the Salisbury nerve agent attack.
The Government's National Security Adviser Sir Mark Sedwill said cyber specialists from the GRU - Russian military intelligence - had targeted Yulia Skripal's email accounts as far back as 2013.
In an account based in part on declassified UK intelligence, Sir Mark also said the Russians had developed a programme in the 2000s to train personnel from "special units" in the use of chemical warfare agents.
Among the techniques they had investigated for delivering nerve agents was applying them to door handles, he said.
The strongest concentration of the Novichok nerve agent found in the Salisbury incident was on the front door handle of Mr Skripal's home.
Russia's ambassador to the UK Alexander Yakovenko described the claims as a "big surprise" and complained they had not been formally notified of the allegations by the UK authorities.
"It is not the right way how the serious people handle these issues," he told a news conference at the embassy in London.
Sir Mark's account - set out in a letter to Nato Secretary General Jens Stoltenberg - represents the most detailed explanation yet by the UK as to why it holds Moscow responsible for the poisoning of the Skripals.
It follows the confirmation on Thursday by the Organisation for the Prohibition of Chemical Weapons that the toxin used in the Salisbury incident was Novichok - a military grade nerve agent developed by Russia in the 1980s.
In his letter, Sir Mark argued that only Russia had the "technical means, operational experience and the motive" to carry out such an attack.
He said Russia had a "proven record of conducting state-sponsored assassination", including the poisoning of Alexander Litvinenko in London in 2006.
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".
"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 cyber specialists," he said.
Sir Mark also identified the key institute for developing Novichok in the former Soviet Union as a branch of the State Institute for Organic Chemistry and Technology at Shikany near Volgograd.
"The code word used for the offensive chemical weapons programme (of which the Novichoks were one part) was FOLIANT," he said.
"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent international weapons controls."
While it was "unlikely" that Novichok could be made and deployed by terrorists or criminal gangs, Sir Mark said Russia had continued to produce and stockpile small quantities of the nerve agent within the last decade.
"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.
"There is no plausible alternative explanation."
Mr Yakovenko however insisted that Russia had destroyed all its chemical weapons and had never possessed Novichok.
"We didn't produce Novichok, we didn't store this Novichok, so-called under the Western classification, was never in our military forces. This is the fact of life," he said.
He also 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. "We cannot be sure that Yulia's refusal to see us is genuine."