A British woman has died after being exposed to the same nerve agent that poisoned a Russian ex-spy and his daughter four months ago, a case that caused a major diplomatic fallout between Moscow and the West.
Here is a timeline of events since Novichok was identified in the poisoning of the Skripals in the southwestern English city of Salisbury in March, and resurfaced in the same area in July in a British couple.
- Slumped on a bench -
Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, are found unconscious on a bench in Salisbury on March 4.
Three days later, with the Skripals in hospital in critical condition, police say the pair were poisoned with a highly-toxic nerve agent.
On March 12, British Prime Minister Theresa May says the military-grade nerve agent has been identified as belonging to the Novichok ("Newcomer") group, which was developed by the Soviet government towards the end of the Cold War.
She says it is "highly likely Russia was responsible". Moscow rejects the claim.
- Diplomatic backlash -
On March 14, London says Moscow is "culpable" and pledges to expel 23 Russian diplomats. It suspends high-level diplomatic contact with Moscow.
The following day, Britain, France, Germany and the United States also blame Russia.
On March 17, Moscow says it will expel 23 British diplomats, among other measures.
On March 20, experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) take samples.
- Mass expulsions -
A raft of Britain's allies announce on March 26 that they are expelling Russian diplomats because of the poisoning.
Washington says it is throwing out 60 Russian "spies" and closing the Russian consulate in Seattle.
By March 27 the number of Russian diplomats expelled from various countries and posts reaches 150.
Two days later Russia announces the expulsion of 60 US diplomats and the closure of the US consulate in Saint Petersburg.
On March 30 it says diplomats from another 23 countries, almost all EU members, will have to leave. Up to 50 more British diplomatic staff will be kicked out, it says.
- Source not verified -
On April 3 the British military facility analysing the nerve agent -- which appears to have been smeared on the front door handle of Skripal's home -- says it has "not verified the precise source" of the substance.
The Kremlin demands an apology and on April 4 its foreign intelligence chief says the case is a "grotesque provocation... concocted by the British and American security services".
Yulia Skripal is released from hospital on April 10.
Two days later, the OPCW confirms Britain's findings on the nerve agent but does not apportion blame.
The next day, National Security Adviser Mark Sedwill says Britain has information that Russian intelligence had been spying on the Skripals for at least five years before the attack.
On April 17, an operation begins to clean up affected areas of Salisbury.
Sergei Skripal is discharged from hospital on May 18.
- Two more poisoned -
On July 4, police reveal that two people are in a critical condition after being found unconscious days earlier in the village of Amesbury, around 12 kilometres (eight miles) from Salisbury.
Later they confirm that the man and woman had come into contact with Novichok.
The situation is declared a "major incident" and counter-terrorism police take the lead in the investigation.
The next day Britain's interior minister Sajid Javid says Moscow needs to explain itself over the use of the toxic agent.
Russia slams Britain for playing "dirty political games" and demands an apology.
- Woman dies -
Police say tests on the couple -- Dawn Sturgess, 44, and Charlie Rowley, 45 -- showed they had handled an unidentified "contaminated item".
They had visited Salisbury the day before they fell ill.
Police announce on July 8 that Sturgess has died.
Expressing shock, May says the incident is being investigated as a murder.
An investigator wearing a protective suit in the English town of Amesbury, where a couple were found in critical condistion after coming into contact with the Novichok nerve agent
How the Novichok nerge agent affects the body's nervous system