Russia’s ambassador to London denied that Moscow had ever produced the nerve agent used in the Salisbury poisoning as a showdown with Britain loomed at the United Nations.

Alexander Yakovenko called Boris Johnson’s assertion that Moscow maintains a stockpile of novichok for assassinations “unacceptable” and “absolutely not true.”

His comments came after security sources told The Times they had pinpointed the location where the nerve agent was made and tested for use in assassinations.

Britain and Russia are preparing for a fresh confrontation at the Security Council after Moscow called an emergency session to demand Britain produce evidence of Russia’s culpability.

Russia lost a vote at the Organisation for the Prohibition of Chemical Weapons in the Hague on Wednesday demanding its experts be involved in…