The pair were discovered slumped on a bench in the city's Maltings shopping centre on 4 March, and are both still critically ill in hospital.
The pair remain unconscious in a critical but stable condition, while Nick Bailey, one of the first police officers on the scene, is now sitting up and talking after initially being admitted to intensive care.
Skripal was a former colonel in Russian military intelligence who was jailed in his country for betraying agents to Britain's MI6 secret service. Six years later, the former colonel was handed over to the U.S. as part of a swap involving espionage suspects.
These highly potent compounds target the enzyme acetylcholinesterase, which breaks down the neurotransmitter acetylcholine to prevent its long-lasting effects at nerve endings.
BRITAIN has multiple options for a response if Russian Federation is proven to be behind a nerve agent attack on a former double-agent this week, according to Russian Federation expert Mathieu Boulegue. Symptoms include disorientation, muscle spasms, breathing difficulties and a general shutdown in body functions that can lead to death if left untreated. The lungs produce mucus, and victims may appear to foam at the mouth.
It had been thought that the policeman may have been exposed to the nerve agent. Higher doses can stop breathing altogether and trigger cardiac arrest.
"British Home Secretary Amber Rudd said Thursday that authorities were avoiding speculation but that Britain "would respond in a robust and appropriate manner" and that it was committed to bringing the perpetrators to justice, 'whoever they are and wherever they may be'".
They can be absorbed through the mouth, nose or skin.
But sources close to the investigation have already ruled out sarin and VX, according to a report from the BBC.More news: What Is International Women's Day Anyway?
Home Secretary Amber Rudd said the use of a chemical weapon on United Kingdom soil was a "brazen and reckless act", and the Government stood ready to act as a clearer picture emerged.
"If it were a standard agent, it would have been identified nearly immediately", says Jean Pascal Zanders, an independent consultant who was previously a senior research fellow at the European Union Institute for Security Studies. However, exactly which chemical agent was used has not yet been disclosed. This nerve agent attack shows that these dangers can be totally unpredictable, and of the most serious kind.
They take different forms including powder and gas but they tend to be a liquid, which can seep through the skin.
Officials say Bailey is making progress.
Citing unnamed security sources, The Sun newspaper said Wednesday they were poisoned by a very rare nerve agent which only a few laboratories in the world could have produced.
Police sergeant, Nick Bailey, 38, is also in a serious condition. For example, the officers may have touched the Skripals' clothing or skin, which could have been contaminated with the nerve agent.
'There's another enzyme in blood called butyrylcholinesterase that is also inhibited by nerve agents and acts as a sort of reservoir, so you can extract the chemical from the enzyme and then do your analysis, ' Hay says.
However, counter-terrorism police, who are leading the investigation, are not making those details public. On 23 November 2006, Litvinenko died in London.
A British inquiry found that Russia was responsible for Litvinenko's killing, and that Russian President Vladimir Putin probably approved it. That finding, rejected by Russian Federation, has stoked diplomatic tensions between the two countries.
Speaking to reporters during a visit to Ethiopia, Lavrov said Russian officials have seen no concrete evidence or "facts" about what happened to Skripal and his daughter.