The letter was made public a day after investigators at the Organization for the Prohibition of Chemical Weapons confirmed British findings that the Skripals were poisoned with a military-grade nerve agent.
It published the unusual allegations in an 8,000-word report on the incident after the Kremlin was accused of using a military grade nerve agent in Salisbury, on March 4.
The report concluded: 'The situation around the Skripals looks more and more like a forcible detention or imprisonment'.
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.
"Only Russia has the technical means, operational experience and the motive".
Last week, the chief executive of the Defense Science and Technology Laboratory in Porton Down, Britain's premier chemical weapons laboratory, said its scientists could not identify "the precise source" of the chemical, though its purity indicated that it was nearly certainly created by a "state actor".
"It is unlikely that Novichok could be made and deployed by non-state actors", he adds.
Tyson Fury return confirmed
If he wants to fight ten fights down the line, he can do that as well, ' the IBF , WBA and WBO champion said. The 29-year-old had not fought competitively since beating Wladimir Klitschko in November 2015.
"This programme subsequently included investigation of ways of delivering nerve agents, including by application to door handles", the letter says.
He also said Russia had a clear motive for attacking Skripal, a former Russian military intelligence officer who had been imprisoned in Russia for spying for Britain only to be set free in a spy swap in 2010.
Skripal, 66, and his daughter, 33, were admitted to a hospital after being found unconscious on a public bench on March 4.
Sir Mark said Russian Federation had a "proven record of conducting state-sponsored assassination" and that it was "highly likely" some defectors may be regarded as "legitimate targets".
Sedwill said "credible open-source reporting and intelligence" showed that in the 1980s the Soviet Union developed a family of nerve agents known as Novichoks at a base in Shikhany near Volgograd.
He said that after the Soviet Union collapsed, Russian Federation signed the Chemical Weapons Convention without reporting its ongoing work on Novichoks.
Such a highly classified documentation is rarely made public and was only released on Friday, after Russian Federation asked to see proofs of the allegations advanced by the UK.
France's Macron agrees with Riyadh on curbing Iran 'expansionism'
The move follows statements by Russian Federation and Syria who both said they wanted to facilitate a visit by inspectors. Macron stressed the need for "respect for worldwide humanitarian law".
Also Friday, Russian Foreign Minister Sergey Lavrov responded to an global chemical weapons watchdog's report on the poisoning.
Meanwhile, Russia's ambassador has accused the British authorities of destroying evidence in the Salisbury nerve agent attack. Her father remains hospitalized but British health officials say he is improving.
Russian Federation has charged that British officials were keeping Skripal's daughter in isolation.
Mr Lavrov said the report only confirmed the composition of the substance and that Britain's claim that it confirmed the United Kingdom position on the Skripal case was overstated.
Russian Foreign Minister Lavrov speaks to the media a joint news conference with Dutch Foreign Minister Halbe Zijlstra following their talks, in Moscow, Russia, Friday, April 13, 2018.
Shortly after the attack in Salisbury, UK Home Secretary Amber Rudd asked police and intelligence agency MI5 to look into further allegations of Russian state involvement in a number of deaths in the UK, prompted by a Buzzfeed report alleging Russian links to 14 suspicious deaths in the UK.
Fake candidates list for Karnataka election sends Congress into tizzy
The first list of BJP, comprising 72 candidates, for the upcoming May 12 Karnataka assembly elections has been released. The meeting discussed the names of probable candidates for rest of the seats in the 225-member state Assembly.
13 April 2018
12 April 2018
11 April 2018
14 April 2018
13 April 2018
12 April 2018
14 April 2018
12 April 2018
11 April 2018
13 April 2018
13 April 2018
12 April 2018