Former Russian double agent Sergei Skripal and his daughter were poisoned with a nerve agent in Salisbury in March last year.
Here is a timeline of how the events unfolded:
March 4 2018
Former Russian double agent Sergei Skripal, 66, and his daughter Yulia, 33, are found unconscious on a park bench in Salisbury.
Police said 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.
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.
DS Bailey is discharged from hospital but says life will "probably never be the same".
Britain’s allies announce 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.
Salisbury District Hospital announces that Ms Skripal has been discharged.
The Department for Environment, Food and Rural Affairs said the Novichok used to attack the Skripals was delivered in a "liquid form".
It is announced that Mr Skripal has been discharged from hospital after more than two months of treatment.
Businesses in the Maltings area of Salisbury reopen following the attack.
Dawn Sturgess and Charlie Rowley fall ill at a flat 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.
Police declare a "major incident" after revealing Ms Sturgess and Mr Rowley have been exposed to an "unknown substance", later confirmed to be Novichok.
New home secretary Sajid Javid accuses the Russian state of using Britain as a "dumping ground for poison" and demands an explanation from the Kremlin for the two episodes.
Forensic investigators in hazardous material suits and gas masks begin searching the building where Ms Sturgess lives.
Ms Sturgess dies in hospital after being exposed to Novichok.
Scotland Yard launch a murder investigation over her death.
Mr Rowley regains consciousness.
Police reveal the Novichok that poisoned them was from a small bottle found in Mr Rowley’s home.
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.
Mr Rowley leaves hospital, telling reporters he feels lucky to be alive.
Ms Sturgess’s funeral service takes place at Salisbury Crematorium.
Russia denounces the imposition of "draconian" new US sanctions after the administration concluded Moscow was responsible for the Salisbury attack.
Independent investigator the Organisation for the Prohibition of Chemical Weapons confirms the toxic chemical which killed Ms Sturgess was the same nerve agent as that which poisoned the Skripals.
Russian President Vladimir Putin says there is "nothing criminal" about Petrov and Boshirov. Downing Street insists they are GRU officers "who used a devastatingly toxic illegal chemical weapon on the streets of our country".
An online investigations group publishes what it calls the true identity of Boshirov, saying he is highly decorated Colonel Anatoliy Chepiga, while a fortnight later Bellingcat says Petrov is a military doctor called Alexander Yevgenyevich Mishkin.
Mr Rowley said he is terrified about his future health.
January 8 2019
Wiltshire Council tells residents there will be four months of deep cleaning and construction work as the Skripals’ former home is to be dismantled, with the roof completely removed.
It emerges 16-year-old Abigail McCourt was the first person to help the Skripals.
The European Union imposes sanctions including travel bans and asset freezes on Russians blamed for the attack.
Mr Rowley’s former home is declared safe.
MPs are told Britain and Russia are in talks to replace the 23 diplomats expelled from the UK in the wake of the attack.
High-ranking Russian military intelligence service officer Denis Sergeev is revealed as being reportedly in Britain during the attack.
The 45-year-old member of the GRU, who uses the alias Sergei Fedotov, arrived 48 hours before the poisoning but it is unclear what role, if any, he played in the attack, the investigative website Bellingcat said
The parents of Ms Sturgess demand justice from the Government.
A huge Russian flag draped across Salisbury Cathedral is branded a "slap in the face".
The Ministry of Defence announces Salisbury is to be declared decontaminated of Novichok after an almost year-long military clean-up of 12 sites.
It is the first anniversary of the attack.