Britain moved to counter Russian propaganda on Salisbury today by releasing new evidence.
The UK authorities disclosed that Moscow security services were spying on the Skripals for at least five years, and hacked the email of Yulia, who was poisoned along with her ex-spy father Sergei last month.
Russia also tested assassination methods including smearing nerve agent on doorhandles - the way the Novichok poison is thought to have been was delivered to the Skripals.
Theresa May's National Security Adviser Mark Sedwill revealed the details in a letter to Nato chief Jens Stoltenberg today.
Mr Sedwill said the information underlined the UK's conclusion that there was 'no plausible' other explanation apart from Russian involvement.
'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. There is no plausible alternative explanation,' he wrote.
The intervention comes as the UK tries to quell a huge Russian misinformation campaign about the attack.
Moscow's ambassador to London, Alexander Yakovenko, launched another furious barrage at a press conference this afternoon - accusing the UK of 'abducting' the Skripals and 'destroying evidence'.
Yesterday the international chemical weapons watchdog backed Britain's findings that military-grade nerve agent had been used against the Skripals.
The respected agency said an analysis of samples from the scene confirmed the UK's assessment that 'high purity' Novichok was used.
Moscow has refused to commit to accepting the findings of the Organisation for the Prohibition of Chemical Weapons (OPCW), and proposed an array of other explanations for the presence of military-grade nerve agent on UK soil.
One of the more outlandish is that it could have been deployed from Britain's own Porton Down laboratory eight miles down the road.
In his letter, Sir Mark set out why the Government believes that only Russia has the 'technical means, operational experience and the motive' to carry out such an attack - including some declassified intelligence material.
He said Russia had a 'proven record of conducting state-sponsored assassination' and that it was 'highly likely' some defectors - like Mr Skripal, a former GRU officer who was exchanged in a spy swap in 2010 - may be regarded as 'legitimate targets'.
'We have information indicating Russian intelligence service interest in the Skripals dating back at least as far as 2013, when email accounts belonging to Yulia Skripal were targeted by GRU cyber specialists,' he said.
Sir Mark also identified the key institute for developing Novichok in the former Soviet Union as a branch of the State Institute for Organic Chemistry and Technology at Shikany near Volgograd.
'The code word used for the offensive chemical weapons programme (of which the Novichoks were one part) was FOLIANT,' he said.
'It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent international weapons controls.'
He said that Russia had continued to produce and stockpile small quantities of Novichoks within the last decade.
'We therefore 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.'
An executive summary of the watchdog's report said: 'The results of the analysis by the OPCW designated laboratories of environmental and biomedical samples collected by the OPCW team confirms the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury and severely injured three people.'
Foreign Secretary Boris Johnson said the OPCW had confirmed Britain's assessment that the substance was 'a military grade nerve agent – a Novichok'.
'This is based on testing in four independent, highly reputable laboratories around the world. All returned the same conclusive results,' he said.
'There can be no doubt what was used and there remains no alternative explanation about who was responsible – only Russia has the means, motive and record.'
Earlier this week Ms Skripal revealed she has refused help from the Russian Embassy.
In a statement issued through the Metropolitan Police, she confirmed that Russia had made contact with her but she did not 'wish to avail myself of their services'.
In the statement, she asked her outspoken cousin Viktoria Skripal, who has suggested Yulia's fiancé was involved in the attack, to refrain from making contact and added that her relative's opinions did not reflect her own.
It comes after dramatic claims from Russia that she and her father had been 'abducted' by the UK which was blocking its offers of assistance.
Miss Skripal added: 'I have access to friends and family, and I have been made aware of my specific contacts at the Russian Embassy who have kindly offered me their assistance in any way they can.
'At the moment I do not wish to avail myself of their services, but, if I change my mind I know how to contact them.
'Most importantly, I am safe and feeling better as time goes by, but I am not yet strong enough to give a full interview to the media, as I one day hope to do.
'Until that time, I want to stress that no one speaks for me, or for my father, but ourselves.
'I thank my cousin Viktoria for her concern for us, but ask that she does not visit me or try to contact me for the time being. Her opinions and assertions are not mine and they are not my father's.'
She once again thanked staff at Salisbury District Hospital for their 'kindness' as she revealed her father remained in their care.
'I have left my father in their care, and he is still seriously ill,' she said. 'I too am still suffering with the effects of the nerve agent used against us.
'I find myself in a totally different life than the ordinary one I left just over a month ago, and I am seeking to come to terms with my prospects, whilst also recovering from this attack on me.
'I have specially trained officers available to me, who are helping to take care of me and to explain the investigative processes that are being undertaken.'
Miss Skripal, 33, and her 66-year-old father were targeted in a poison plot in Salisbury five weeks ago, but the daughter has now been discharged from hospital.
It is believed that British authorities immediately spirited Miss Skripal away to a secure location when she was discharged from hospital.
The Russian embassy reacted angrily, suggesting in a series of tweets that the Russian national had been taken against her will.
Former double agent Mr Skripal was jailed in Russia for selling secrets to MI6 but was released as part of a spy swap deal in 2010 and settled in the UK.
It is hoped he will soon be fit for release from hospital, despite grave fears that the exposure to military-grade nerve agent Novichok on March 4 would prove fatal.
Britain has said Russian state involvement is the only plausible explanation for the attack and has led a worldwide reaction involving the expulsion of more than 100 diplomats.