Earlier on Friday, the national security adviser to Britain's prime minister told NATO Secretary General Jens Stoltenberg in a letter that email accounts of Yulia had been targeted in 2013 by cyber specialists from Russia's GRU military intelligence service.
By 1993, when Russia signed the Chemical Weapons Convention, Sedwill said it was "likely" that some Novichoks had passed testing to allow their use by the Russian military.
Moscow has suggested Britain carried out the attack to stoke anti-Russian hysteria, but Britain asked the Organisation for the Prohibition of Chemical Weapons (OPCW) to check samples from Salisbury.
Mr. Skripal remains hospitalized almost five weeks after he was poisoned, but his daughter, Yulia, has recovered and was moved to a secure location this week.
Richard Guthrie, an independent chemical-weapons expert, says an important detail in the investigation is that the toxic substance is of "high purity".
"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", the letter says.
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 Russian Federation in the 1980s.More news: Apple Inc. loses a patent infringement lawsuit against VirnetX
"It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassination", he said.
He said Russian Federation had a proven record of state-sponsored assassinations and had tested ways of delivering chemical weapons, including the use of door handles to spread nerve agents, as Britain believes was done in the Skripal case.
In a news conference Friday afternoon, the Russian ambassador to Britain, Aleksandr V. Yakovenko, dismissed the letter and "all these allegations" surrounding the nerve agent attack as having "nothing to do with reality". He said it was highly unlikely that any former Soviet republic besides Russian Federation pursued an offensive chemical weapons program.
In his letter, Sir Mark set out why the United Kingdom believes that only Russian Federation has the "technical means, operational experience and the motive" to carry out such an attack - including some declassified intelligence material. "The Russian state has previously produced Novichoks and would still be capable of doing so".
"We get the impression the British Government is deliberately pursuing the policy of destroying all possible evidence".
He also complained at the continued refusal of the British authorities to grant consular access to Ms Skripal following her discharge from hospital.
"I want to underline: the OPCW only has confirmed the composition of the chemical agent", he said. Her father remains hospitalized but British health officials say he is improving.