Among major producers of the toxic chemicals from the so-called “Novichok” family were the UK (70 chemicals), the US (19 chemicals) and other countries. All this information was submitted to the OPCW.
Novichok is known to be more powerful than VX and was developed in Russia in the 1970s and 1980s www.chemistryworld.com/news/russian-novichok-nerve-agent-linked-to-attack-on-ex-spy/3008773.article,
It's so unusual, that very few scientists outside of Russia have any real experience in dealing with it and no country outside of Russia is known to have developed the substance https://edition.cnn.com/2018/03/13/europe/what-is-novichok-nerve-agent-intl/index.html www.bbc.com/news/world-europe-43377698, , www.theguardian.com/world/2018/mar/14/nerve-agent-novichok-produced-russia-site-expert,
A Russian scientist who originally helped develop the nerve agent states that other countries could have had test samples, but production was only refined in the U.S.S.R. and Russia www.cbrneworld.com/_uploads/download_magazines/09_summer_The_agent_fate_of_Novichok.pdf,
In its statement, the European Union expressed shock at the offensive use of any military-grade nerve agent, of a type developed by Russia, for the first time on European soil in over 70 years. http://www.consilium.europa.eu/en/press/press-releases/2018/03/19/statement-by-the-foreign-affairs-council-on-the-salisbury-attack/