Ex-Russian spy Sergei Skripal, 66, and his 33-year-old daughter Yulia were found slumped on a bench in Salisbury town centre on March 4, exposed to the deadly nerve agent Novichok.
The Kremlin strongly denied any involvement in the attack but the UK has remained adamant that Russia was responsible.
What ensued were several bouts of mudslinging between Moscow, London and its allies as an investigation was opened into the incident.
In a concatenation of events, 29 nations expelled Russian diplomats over the poisoning and Russia told the UK that more than 50 of its diplomats had to leave the country.
Here's what we know about the attack and what remains unanswered.
Where did the poison come from?
Porton Down laboratory, a UK defence research facility, said Tuesday that the precise source of the nerve agent used in Salisbury has not been verified.
The laboratory's chief executive Gary Aitkenhead quashed claims made by Russia’s ambassador to the EU, Vladimir Chizhov, that the chemical came from the British-based lab.
In a television interview Chizhov said: “When you have a nerve agent or whatever, you check it against certain samples that you retain in your laboratories.
"And Porton Down, as we now all know, is the largest military facility in the United Kingdom that has been dealing with chemical weapons research. And it's actually only eight miles from Salisbury."
Aitkenhead retorted that the military laboratory had the "highest levels of controls of security".
He said the lab had provided scientific information to the UK government which then "used a number of other sources to piece together the conclusions".
"It is our job to provide the scientific evidence of what this particular nerve agent is — we identified that it is from this particular family and that it is a military grade but it is not our job to say where it was manufactured," he added.
The British government dismissed Chizhov's claims as “nonsense,” with a government spokesperson maintaining Russia was responsible, adding there was "no other plausible explanation".
Who administered the nerve agent?
Porton Down laboratory identified the substance used in the attack as Novichok, saying it was likely to have been deployed by a "state actor".
The UK said it believed Russia was responsible, citing further intelligence — which Downing Street did not disclose — as the grounds for this claim, an assertion that it maintains.
Russia's foreign ministry responded in a statement on March 28 saying: “An analysis of all the circumstances… leads us to think of the possible involvement in it (the poisoning) of the British intelligence services.”
"If convincing evidence to the contrary is not presented to the Russian side we will consider that we are dealing with an attempt on the lives of our citizens as a result of a massive political provocation.”
Where were the Skripals poisoned?
British counter-terrorism police said on March 28 that the military-grade nerve toxin was left on the front door of their home in Salisbury.
“Specialists have identified the highest concentration of the nerve agent, to date, as being on the front door of the address,” Scotland Yard said in a statement.
Public health concern surrounding Zizzi restaurant and the Mill pub, both locations that the Skripals visited before they were found, could suggest they came into contact with Novichok prior to the time they arrived at those places.
President Vladimir Putin described the events as an "anti-Russia" campaign and said he hoped "a line can be drawn under" the incident at a meeting on Wednesday.
He added that Russia wanted to be involved in the investigation.
The Organisation for the Prevention of Chemical Weapons (OPCW), the world's chemical weapons watchdog, was to meet in the Hague to discuss the poisoning, at the Kremlin's request.
The UK Foreign Office said it was a "diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion".
The OPCW said it should receive results of its own independent tests within a week and while the organisation does not have the authority to attribute blame, it could request access to former Soviet Union facilities to check all chemical weapon stockpiles were destroyed.