Russian military intelligence targeted Salisbury poisoning victim Yulia Skripal’s email account as early as 2013, according to the UK government’s national security adviser.
In a letter to Nato’s secretary general, Sir Mark Sedwill said there was also evidence Russia trained “special units” to apply nerve agents.
He said the methods included putting nerve agents on door handles, as police say happened in the attack on Yulia, 33, and her ex-spy father Sergei on 4 March.
The letter was sent ahead of an invitation for Sir Mark to brief Nato on 15 April about the Salisbury attack.
It comes after the international chemical weapons watchdog confirmed the UK’s analysis of the type of nerve agent used in the Russian ex-spy poisoning.
The UK says Ms Skripal and her 66-year-old father, who were found slumped on a park bench in the city, were exposed to the toxic nerve agent Novichok.
The Russian government has denied any involvement and accused the British of inventing a “fake story”.
Sir Mark’s letter says the government “continue to judge that only Russian 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 Skripal remains at Salisbury District Hospital. His daughter was discharged on Monday.