The US has announced it will impose sanctions on Russia after concluding that Moscow was responsible for the Novichok nerve agent attack in Salisbury.
Former spy Sergei Skripal and his daughter Yulia were poisoned in the Wiltshire town in March.
Here, the Press Association looks at how events have unfolded.
Saturday, March 3 2018
Ms Skripal lands at Heathrow at 2.40pm.
Mr Skripal, 66, and his daughter, 33, are found unconscious on a park bench in Salisbury.
Mr Skripal and his daughter are publicly identified and declared “critically ill” in hospital.
Police say a nerve agent was used to poison the pair, and the case is being treated as attempted murder.
Then home secretary Amber Rudd says a Wiltshire police officer, Detective Sergeant Nick Bailey, is seriously ill in hospital.
Up to 500 diners and pub-goers are told to wash clothes and other items after traces of a nerve agent are found in The Mill pub and the nearby Zizzi restaurant – venues visited by the Skripals.
The Russian Embassy in the UK says Moscow “will not respond to London’s ultimatum”. It emerges that 38 people received medical treatment in the aftermath of the attack.
Mrs May tells MPs the UK will expel 23 Russian diplomats, calling the incident an “unlawful use of force by the Russian state against the UK”.
Leaders of Britain, the US, Germany and France issue a joint statement blaming Russia for the attack.
Russia announces the expulsion of 23 UK diplomats and says it will shut down the British Council and British Consulate in St Petersburg.
Det Sgt Bailey is discharged from hospital, but says in a statement: “Normal life for me will probably never be the same.”
Britain’s allies announce that more than 100 Russian agents are being sent home from 22 countries in what Mrs May calls the “largest collective expulsion of Russian intelligence officers in history”.
Scotland Yard reveals Mr Skripal and his daughter first came into contact with the nerve agent at his home.
The head of the Porton Down military research facility says his scientists have not verified that the nerve agent used in Salisbury came from Russia.
The hospital announces Ms Skripal has been discharged from hospital.
The Department for Environment, Food and Rural Affairs says the Novichok used to attack the Skripals was delivered in a “liquid form”.
It is announced Mr Skripal has been discharged from Salisbury District Hospital after more than two months of treatment.
Businesses in the Maltings area of Salisbury reopen following the attack.
Wiltshire Police’s response to the attack in Salisbury is expected to cost £7.5 million.
Homes, cars and possessions belonging to Mr Skripal and Det Sgt Bailey will be bought by taxpayers in a £1 million deal, it is reported.
Britain has exterminated evidence related to the poisonings and has benefited politically from the incident, Russia’s foreign minister Sergey Lavrov claims.
Dawn Sturgess and Charlie Rowley fall ill at a property in Muggleton Road in Amesbury – eight miles from Salisbury – and are taken to hospital.
Wiltshire Police warn of the dangers of contaminated drugs after the couple fall ill. Detectives believe they may have taken heroin or crack cocaine. The pair are in a serious condition at Salisbury District Hospital.
Later that night, Scotland Yard reveals that tests show the couple have been exposed to Novichok. Police are unable to say whether it was the same batch that the Skripals were exposed to.
Home Secretary Sajid Javid accuses the Russian state of using Britain as a “dumping ground for poison”, and demands that the Kremlin provide an explanation for the two episodes.
Forensic investigators in hazardous material suits and gas masks begin searching the building where Ms Sturgess lives.
A police officer attends hospital over concerns of a possible exposure to Novichok – he later tests negative for the nerve agent.
Mr Javid, on a visit to Wiltshire, says there will be no fresh sanctions against Russia over the attacks.
Ms Sturgess dies in hospital following her exposure to Novichok.
Scotland Yard launches a murder investigation over the death of Ms Sturgess.
Mr Rowley regains consciousness.
Investigators speak to Mr Rowley in hospital.
Specialist officers search Queen Elizabeth Gardens in Salisbury, a park at the centre of the poisoning probe.
Police are believed to have identified the suspected perpetrators of the Skripal attack.
An inquest opens into the death of Ms Sturgess. Her body is formally released to her family for her funeral.
Mr Rowley is discharged from Salisbury District Hospital after almost three weeks of treatment.
The funeral of Ms Sturgess is held at Salisbury Crematorium.
The US announces it will impose new sanctions on Russia in response to the nerve agent attack in Salisbury, issuing a a formal determination that Russia violated international law by poisoning the Skripals.
The British Government welcomes the action, saying that it sends “an unequivocal message to Russia that its provocative, reckless behaviour will not go unchallenged”.