Russian intelligence agencies targeted poison victim Yulia Skripal’s email account as early as 2013, the UK’s national security adviser has said.
In a letter to Nato, Sir Mark Sedwill said Russia trained “special units” to use nerve agents, including the method of applying them to door handles.
Police say the highest concentration of nerve agent Novichok was on the front door of Mr Skripal’s Salisbury home.
Sir Mark is due to brief Nato on Sunday about the 4 March Salisbury attack.
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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, 33, and her 66-year-old father, who were found slumped on a park bench in the city on 4 March, were exposed to the toxic nerve agent Novichok.
The Russian government has denied any involvement and accused the British of inventing a “fake story”.
The Russian embassy, which is publishing its own report into the Salisbury attack, said the UK had not produced “any evidence” to support its claims about the incident.
Its ambassador Alexander Yakovenko called Britain’s response “quite strange, to put it mildly” as he criticised the UK’s refusal to grant consular access to Ms Skripal.
Mr Skripal remains at Salisbury District Hospital, while his daughter was discharged on Monday.
Sir Mark’s letter to Nato Secretary General Jens Stoltenberg is the first time the UK government has claimed to have specific information that the Russians were interested in the Skripal family.
But the details contained in the letter do not identify the exact culprits behind the poison attack.
The letter accuses Russia of testing assassination methods with chemical weapons, including using nerve agents smeared on to door handles.
Traces of Novichok have been found at sites across Salisbury at low concentrations following the attack – with the highest concentration of the nerve agent found on the Skripals’ door handle.
No 10 has released a letter from National Security Advisor Mark Sedwill to NATO Secretary-General. It spells out more details of intelligence pointing to Russia in the Salisbury case. Includes suggestion that Russia was specifically looking at putting nerve agents on door handles pic.twitter.com/hd7Yu4z6y8
— Daniel Sandford (@BBCDanielS) April 13, 2018
End of Twitter post by @BBCDanielS
He said it was “highly likely” people such as Mr Skripal, who was exchanged in a spy swap in 2010 after being convicted of passing information to the UK, may be regarded as “legitimate targets” for “state-sponsored assassination” by Russia.
Sir Mark says the UK government continues “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”.
“There is no plausible alternative explanation,” he added.
The letter also claims President Vladimir Putin was closely involved in the Russian chemical weapons programme in the mid-2000s, and that it was “highly unlikely” any former Soviet republic other than Russia pursued chemical weapons after independence.
Meanwhile, Metropolitan Police Commissioner Cressida Dick has said around 250 detectives were working on the Salisbury investigation.
She said they would do everything to establish the facts and “if at all possible” bring those responsible to justice.