In a separate interview, British Foreign Secretary Boris Johnson responded by accusing Russia of secretly stockpiling the deadly nerve agent and also studying how to use the toxin for assassinations.

BORIS JOHNSON BRITISH FOREIGN SECRETARY "And I think listening to the Russian response to this again, to the response of the Russian ambassador to the EU with his satirical suggestion that this was done by UK agents from Porton Down. This is not the response of a country that really believes itself to be innocent. This is not the response of a country that really wants to engage in getting to the bottom of the matter."

The tit-for-tat battle has seen London and Moscow each expel 23 diplomats over the nerve agent attack on Sergei Skripal, and his daughter Yulia. Officials from the world's chemical weapons watchdog will arrive in Britain on Monday to investigate the samples used in the attack. The British foreign ministry says the results should come out in about two weeks.