Former British Prime Minister Theresa May lied when she claimed that the Novichok nerve agent used in the Skripal poisoning was made in Russia. The British lab in Porton Down denied that the poison was made in Russia.
This is a recurring pro-Kremlin disinformation narrative about the Skripal poisoning in the UK, attempting to divert attention from Russia's proven responsibility for the attack. Several conspiracy theories have also focused on the Porton Down laboratory. Earlier, for example, pro-Kremlin media have claimed that Porton Down might have manufactured Novichok, and that Porton Down was responsible for the Amesbury poisoning.
As clearly stated by Ambassador Karen Pierce, UK Permanent Representative to the UN, at the Security Council on the 5th of September 2018, UK police and UK intelligence services have produced sufficient hard forensic evidence to charge two Russian nationals, identified as officers of the Russian Military Intelligence, GRU, for the attack against the Skripals. The investigative outlet Bellingcat has later revealed the identities of the two Russian nationals.
Furthermore, the UK investigation found that Sergey and Yuliya Skripal were poisoned using a specific Novichok nerve agent that could not have been produced by non-state actors. This was confirmed by an independent OPCW [Organisation for the Prohibition of Chemical Weapons] analysis.
According to the UK intelligence assessment, based on open-source analysis and intelligence information, in the past decade, Russia has produced and stockpiled small quantities of Novichok agents, long after it signed the Chemical Weapons Convention.
For similar cases see here.