Russia’s intelligence agencies spied on former double agent Sergei Skripal and his daughter Yulia for at least five years before they were attacked with a nerve agent in March, The Guardian reported.

UK’s national security adviser, Mark Sedwill said in a letter to NATO Secretary General Jens Stoltenberg on Friday that email accounts of Yulia had been targeted in 2013 by cyber specialists from Russia’s GRU military intelligence service.

According to some information from the UK authorities, Russia had tested whether door handles could be used to deliver nerve agents and had targeted the email accounts of Sergei and Yulia Skripal since at least 2013.

It was noted that Russia had continued developing small amounts of novichok over the past decade.

“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,” he said.

“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 [Vladimir] 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).”