Princeton: A Russian chemist who helped develop the Soviet-era nerve agent used to poison a former Russian double agent in southern England says only the Russian government could have carried out the attack with such a deadly and advanced toxin.

Vil Mirzayanov, 83, said he had no doubt that Russian President Vladimir Putin was responsible, given that Russia maintains tight control over its Novichok stockpile and the agent is too complicated for a non-state actor.

"The Kremlin all the time, like all criminals, denying - it doesn't mean anything," Mirzayanov said in an interview in his home in Princeton, New Jersey, where he has lived in exile for more than 20 years.

Sergei Skripal, a former Russian military intelligence agent who betrayed dozens of Russian agents to British intelligence, and his daughter are fighting for their lives after they were found on March 4 collapsed on a bench, having been poisoned with Novichok, according to British authorities.