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Britain’s top National Security chief has revealed further evidence that Russia was behind the nerve agent attack in Salisbury.

In a letter to NATO Secretary General Jens Stoltenberg, National Security Advisor Mark Sedwill said Sergei and Yulia Skripal had been under scrutiny from Russian intelligence services in recent years.

Yulia Skripal’s email account, he said, was targeted by Russian intelligence cyber experts as far back as 2013.

The letter, shared with NATO allies, set out the government’s case for Russia being the only state with “the technical means, operational experience and the motive” to carry out the attack.

Mr Sedwill wrote: “There is no plausible alternative explanation”.

(Image: AFP) (Image: AFP)

The letter revealed Russia had “produced and stockpiled” small quantities of nerve novichok nerve agents, and had tested ways to deliver them by applying them to door handles during the 2000s.

The letter notes the highest concentrations of nerve agent found in Salisbury were on the handle of Mr Skripal’s front door.

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British intelligence believes Vladimir Putin himself was closely involved in Russia's chemical weapons programme during the mid-2000s.

Yesterday, international chemical weapons watchdog the Organisation for the Prohibition of Chemical Weapons, published a report confirming the chemical used in the attack was novichok.

But it was not the watchdog’s role to establish who produced or deployed it.

The letter reveals the Soviet Union first developed novichok class nerve agents in the 1980s, in a chemical weapons programme known as FOLIANT.

And it claims Russia had already approved the use of novichoks by the military before they signed the Chemical Weapons Convention in 1993, and continued to develop them afterward.

The OPCW’s analysis found the novichok used in the attack was almost entirely pure.

(Image: REX/Shutterstock)

In the letter, Mr Sedwill wrote that it was highly unlikely that any former Soviet republic other than Russia had pursued an offensive chemical weapons programme after independence, and that “it is unlikely that novichoks could be made and deployed by non-state actors (eg a criminal or terrorist group) especially at the level of purity confirmed by OPCW.”

And he noted the Russian state’s history of carrying out state sponsored assassination against opponents of Vladimir Putin, including the murder of former spy Alexander Litvinenko.

Setting out the motive for the attack, Mr Sedwill wrote: “It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassination.

"We have information indicating Russian intelligence service interest in the Skripals, dating back at least as far as 2013, when e-mail accounts belonging to Yulia Skripal were targeted by GRU (Russian military intelligence) cyber specialists.”

Full text of the letter from Mark Sedwill to NATO Secretary General Jens Stoltenberg

(Image: PA)

Thank you again for your invitation to me to brief the North Atlantic Council on 15 March regarding the recent attack in Salisbury.

I am pleased that we have been able to remain in close contact with you and NATO Allies following this attack, and particularly grateful for the measures taken by you and many Allies in response.

As you will be aware, yesterday the Organisation for the Prohibition of Chemical weapons published their report summarising the analysis of environmental and biomedical samples relating to the investigation into the attempted assassination of Mr Skripal and his daughter.

As signatories to the Chemical Weapons Convention, all NATO Allies have received the full report, and several will take part in next Wednesday’s meeting of the OPCW Executive Council which the UK has called.

The OPCW’s analysis matches the Defence Science and Technology Laboratory’s own, confirming once again the findings of the United Kingdom relating to the identity of the toxic chemical of high purity that was used in Salisbury. OPCW have always been clear that it was their role to identify what substance was used, not who was responsible.

I would like to share with you and Allies further information regarding our assessment that it is highly likely that the Russian state was responsible for the Salisbury attack. Only Russia has the technical means, operational experience and the motive.

First, the technical means. DSTL scientific analysis found that Sergey and Yulia Skripal were poisoned using a specific Novichok nerve agent. OPCW’s analysis confirmed the findings of the United Kingdom relating to the identity of the toxic chemical.

This was found in environmental samples taken at the scene and in biomedical samples from both Skripals and Police Sergeant Nick Bailey, the first responder. DSTL established that the highest concentrations were found on the handle of Mr Skripal’s front door.

These are matters of fact. But, of course, the DSTL analysis does not identify the country or laboratory of origin of the agent used in this attack.

A combination of credible open-source reporting and intelligence shows that in the 1980s the Soviet Union developed a new class of ‘fourth generation’ nerve agents, known as Novichoks.

The key institute responsible for this work was a branch of the State Institute for Organic Chemistry and Technology at Shikhany near Volgograd.

The codeword for the offensive chemical weapons programme (of which Novichoks were one part) was FOLIANT.

It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent international chemical weapons controls. The Russian state has previously produced Novichoks and would still be capable of doing so.

Russia’s chemical weapons programme continued after the collapse of the Soviet Union. By 1993, when Russia signed the Chemical Weapons Convention (CWC), it is likely that some Novichoks had passed acceptance testing, allowing their use by the Russian military.

Russia’s CWC declaration failed to report any work on Novichoks. Russia further developed some Novichoks after ratifying the convention.

In the mid-2000s, President Putin was closely involved in the Russian chemical weapons programme.

It is highly unlikely that any former Soviet republic (other than Russia) pursued an offensive chemical weapons programme after independence.

It is unlikely that Novichoks could be made and deployed by non-state actors (eg a criminal or terrorist group), especially at the level of purity confirmed by OPCW.

Second, operational experience. Russia has a proven record of conducting state-sponsored assassination.

The Owen Report from the UK’s public inquiry into the death of Aleksandr Litvinenko concluded in January 2016 that he was deliberately poisoned with Polonium 210, that there was a ‘strong probability’ that the FSB directed the operation, and that President Putin ‘probably approved it’.

Commenting on other suspected assassinations between 2002-2006 Sir Robert Owen wrote: “These cases suggest that in the years prior to Mr Litvinenko’s death, the Russian state may have been involved in the assassination of Mr Putin’s critics” and that “the Russian state may have sponsored attacks against its opponents using poisons”.

Since 2006, there have been numerous suspected Russian state-sponsored assassinations outside the former Soviet Union.

During the 2000s, Russia commenced a programme to test means of delivering chemical warfare agents and to train personnel from special units in the use of these weapons.

This programme subsequently included investigation of ways of delivering nerve agents, including by application to door handles.

Within the last decade, Russia has produced and stockpiled small quantities of Novichoks under the same programme.

Third, the motive. Sergey Skripal was a former Russian military intelligence (GRU) officer, convicted of espionage in 2004. It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassination.

We have information indicating Russian intelligence service interest in the Skripals, dating back at least as far as 2013, when e-mail accounts belonging to Yulia Skripal were targeted by GRU cyber specialists.

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.

I would of course be pleased to brief you or NATO Allies further regarding this attack.

I know that NATO will remain seized of the need to confront the increasingly aggressive pattern of Russia behaviour of which the attack in Salisbury was an acute and recent example.

I am copying this letter to the Delegations of all NATO Allies as well as the Delegations of other EU Member states to NATO.

I will also send a copy to the Office of the High Representative of the European Union for Foreign Affairs and Security Policy.

MARK SEDWILL