Russia's UN ambassador has insisted that development work on Soviet-era nerve agents stopped in 1992, and that existing stockpiles were destroyed in 2017.
In September 2017, the Organisation for the Prohibition of Chemical Weapons (OPCW) confirmed the full destruction of the 39,967 metric tons of chemical weapons possessed by Russia.
Moscow has denied any involvement in the Skripals' poisoning and demanded proof.
A former head of Russia's secret police told local media that Novichok could have been stored in any of the former Soviet states - pointing the finger at Ukraine.
A sample is to be sent to the OPCW - the independent international body set up to stop chemical warfare - for analysis.
It called the use of a chemical agent in Salisbury "extremely worrying" in a tweet, adding that "those found responsible for this must be held accountable for their actions".
Chemical weapons expert Hamish de Bretton-Gordon called for the OPCW to be allowed to visit the Russian town where he alleged the nerve agent was made.
"The only other people who could perhaps make Novichoks, if they had the blueprint to do it, would be [DSTL at] Porton Down in the UK, perhaps the US, maybe the French," he said,
but added, "I have it on very good authority that Novichoks were only ever made in Shikhany in central Russia."