It was developed in secret by the Soviet Union during the Cold War in the 1980s as a means of countering USA chemical weapons defenses, but was revealed to the world by former Soviet scientist and whistleblower Vil Mirzayanov.
A middle-aged British woman, who somehow came into contact with the Soviet-era nerve agent known as Novichok, died Sunday evening at a hospital in south England where she was being treated for exposure to the military-grade chemical weapon.
Police suspect Rowley and Sturgess handled a discarded item from the first attack, though they have not determined for certain that the two cases are linked.
Russian Federation hit back, denouncing Britain for playing "dirty political games".
Assistant Commissioner Neil Basu, the head of UK Counter Terrorism policing, said 100 detectives are working as quickly as possible to identify the source of the recent contamination.
"The 45-year-old man who fell ill with Dawn remains critically ill in hospital and our thoughts are with him and his family as well".
Britain maintains the March attack on the Skripals had been ordered by the Russian government, a charge denied by representatives of Russian President Vladimir Putin.
The Britons were initially thought to have taken an overdose of heroin or crack cocaine.
Peskov went on to comment on a separate statement by Russia's representative at the Organization for the Prohibition of Chemical Weapons (OPCW), Alexander Vasilievich Shulgin.
Specialists at the nearby Defense Science and Technology Laboratory at Porton Down, a military research facility, identified the agent used against the Skripals, Sturgess and Rowley as Novichok.
Javid said earlier on Sunday that police had a working hypothesis that the two poisoning incidents were connected.
Senior Russian lawmaker Sergei Zheleznyak, a member of the ruling United Russia party, also accused Britain of trying to sully the July 16 summit. He remains in a critical condition.
Sergei and Yulia Skripal were both discharged from hospital in May and have been offered permanent close protection - as well as a possible settlement in the US or Australia. Her father was discharged on May 18. She said: "The staff here at Salisbury District Hospital worked tirelessly to save Dawn". "They did everything they could", she said.
Alastair Hay, a professor of environmental toxicology at Leeds University, said the hospital probably now had more experience than anywhere else in the world with Novichok cases, but there were limits to what doctors could do.
Two days later, three days after the attack, the police confirmed the use of nerve agent.
He said no one else in the Amesbury and Salisbury region where the couple lived in southwestern England has shown any sign of Novichok poisoning, with over 20 people who had specifically screened for Novichok exposure cleared.