The British government has set out its most detailed explanation yet of why it believes Russia is responsible for the attempted assassinations of former Russian spy Sergei Skripal and his daughter, Yulia.

Sir Mark Sedwill, the prime minister's national security adviser, said Russia had been testing nerve agents on door handles – which is how police suspect the pair were poisoned – and email accounts belonging to Yulia Skripal had been targeted by Russian intelligence as far back as 2013.

In a letter to NATO secretary general Jens Stoltenberg on Friday, Sedwill said: "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."

It comes a day after the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW), confirmed the UK's findings that the Skripals were poisoned by the Novichok nerve agent in Salisbury on 4 March.



Sergei Skripal remains in hospital in serious condition, while Yulia Skripal has been released. A police officer, detective sergeant Nick Bailey, was inadvertently poisoned after tending to the pair, but was released from hospital on 22 March.

In the letter, Sedwill said Novichok was found in environmental samples taken at the scene and in biomedical samples from both Skripals and DS Bailey, and that the "highest concentrations were found on the handle of Mr Skripal’s front door".

He said: "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."