Novichok is a deadly Soviet-era nerve agent that came to prominence following the near-fatal poisoning in Salisbury of former Russian spy Sergei Skripal and his daughter Yulia, last March.
It is believed the nerve agent is also responsible for the poisoning of a British couple who may have come into contact with remnants of the Novichok.
Novichok was the product of a highly secretive Soviet chemical weapons programme. The programme was used to design a new generation of chemical weapons called Foliant and began in the 1970s. One of the reasons for establishing the programme was for the Soviet Union to counter the latest US chemical weapons at the time.
Soviet leaders wanted the equivalent of US binary weapons, which were agents made up of relatively harmless components. These components then turned deadly when mixed, making them easier to operate than regular chemical weapons.
While agents of the Novichok class were highly lethal, the programme was only partly successful. It was found that some of the components were as toxic as military-grade nerve agents and were therefore hard to handle safely.
The Soviet leadership eventually lost interest in chemical weapons, as they felt the weapons were excessive, especially when compared with Moscow's massive nuclear arsenal. Novichok-class agents were only produced in lab quantities and never entered production. Vladimir Uglev, who was a top scientist in the programme, estimates that about 100 kilograms were made for research and military tests.
Uglev said he was the first to synthesise A-234, which is the strain of the Novichok family of agents used in the attack that nearly killed Skripal and his daughter. Uglev described the agent as being much deadlier than any US equivalent.
Just a few milligrams of the odourless liquid, which is about the weight of a snowflake, were enough to kill a person within minutes.
Uglev said A-234 could remain deadly for a long time, even if a few tiny drops are left in a syringe or impregnated into wood.
The main Soviet research centre that designed the Novichok-class agents was located in Shikhany, a town in southwestern Russia. It was one of the "closed cities" isolated by the KGB. The sprawling facility was also home to chemical depots and a military firing range, where nerve agents were tested.
Some Novichok-related research was also conducted at the main Moscow research centre, which shared samples with other labs across the Soviet Union.
Despite the US oversight to dismantle Russia's chemical arsenals after the Soviet collapse, Uglev said he could not exclude the possibility that some lab workers may have been tempted to sell toxic substances amidst the economic meltdown and political turmoil in the 1990s.
According to AP, Uglev and other experts said it may never be possible to determine the nerve agent's origin. To determine what specific lab produced a given sample of Novichok, it is necessary to find an identical specimen from the same batch, which is an impossible task. "You can identify the agent, but it's impossible to track down its source," Uglev said.
Russia has fiercely denied British accusations over the Skripals' poisoning. Instead, Russia has accused London of using the incident to fan an anti-Russian campaign. Moscow said that last year it completed the destruction of some 40 000 metric tons of chemical weapons, which had been left over from the Soviet era. This was part of an effort that spanned two decades and was performed under close international oversight.
Asked to comment on the second case of Novichok poisoning in England, President Putin's spokesman, Dmitry Peskov, reaffirmed the Kremlin's staunch denial of any involvement. "Russia has categorically denied and continues to categorically deny the possibility of any kind of involvement in what was happening there," he said.
Russian Foreign Ministry spokeswoman Maria Zakharova has urged the British government to end what she described as "intrigues and games with chemical agents." Some Russian lawmakers have even suggested that the poisoning could be traced to a British source.