Mr Skripal, 66, who was poisoned with a nerve agent on March 4 along with his 33-year-old daughter, Yulia, was left fighting for his life in hospital after being found unconscious on a park bench in Salisbury.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said the chemical used against Sergei and Yulia Skripal was "of high purity", without mentioning the exact name of the substance which will be reserved for the complete classified report it has prepared. "This programme subsequently included investigation of ways of delivering nerve agents, including by application to door handles". The Skripals are thought to have come into contact with the nerve agent on their front door. "That was a military- grade nerve agent - a Novichok", said United Kingdom foreign secretary Boris Johnson.
Britain's security chief revealed that Russian Federation spied on the former double agent Sergei Skripal in the five years before he and his daughter were attacked with novichok, a type of nerve agent, last month.
In the letter, Sedwill said only Russian Federation had the "technical means, operational experience and the motive" for the attack. The letter read: 'It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassination.
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. The key institute responsible for this work was a branch of the State Institute for Organic Chemistry and Technology at Shikhany defence laboratory, near Volgograd in south-west Russian Federation.
"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent worldwide chemical weapons controls", Sir Mark wrote.More news: Saturday event seeks to raise awareness about child abuse
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"We didn't produce Novichok, we never have - that's the official position", he said.
Skripal confirmed that she was safe and had specially trained officers available to her during the recovery. The alleged hackers' attack is described as retaliation against London accusing Moscow of being involved in the Skripal case. This has also prevented it from acting on the use of chemical weapons in Syria. "Within the last decade, Russian Federation has produced and stockpiled small quantities of Novichoks under the same programme", he said.
The poisoning of Skripal, a former double agent who settled in Britain in 2010 after being released in a spy swap, showed "how reckless Russian Federation is prepared to be", the head of Britain's GCHQ spy agency said.
She also said she would give interviews to the media in time, but asked the press to have patience while she recovers.
The Russian embassy in London cast doubt on the authenticity of Yulia Skripal's statement, saying it indicates the UK's "forcible isolation of the Russian citizen".