Russian intelligence agencies had been spying on Sergei and Yulia Skripal for at least five years before they were poisoned with a nerve agent, Britain's National Security Adviser Mark Sedwill said on April 13.
The Skripals were found unconscious on March 4 in the southern English city of Salisbury on March 4. Britain has blamed Russia for the attempted murder -- a charge that Moscow fiercely denies.
In a letter to NATO Secretary-General Jens Stoltenberg, Sedwill said that only Russia has the "technical means, operational experience, and the motive" for the attack against the former Russian double agent and his daughter.
The Soviet Union developed fourth-generation nerve agents known as Novichoks in the 1980s, Sedwill said.
The agents were developed at the State Institute for Organic Chemistry and Technology near Volgograd under the codeword FOLIANT.
"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent international chemical weapons controls," Sedwill said in the letter.
After the demise of the Soviet Union, Russia signed the Chemical Weapons Convention without reporting its ongoing work on Novichoks, Sedwill said, adding that it was highly unlikely that any former Soviet republic besides Russia pursued such military research.
The Organization for the Prohibition of Chemical Weapons (OPCW) on April 12 confirmed Britain's findings about the nerve agent used in the attack after testing samples from Salisbury.