Theresa May will sit down with Russian president Vladimir Putin and call for the suspects in the Salisbury nerve agent attack to face justice.
Here are some of the key events in the case.
– March 4 2018
Former Russian double agent Sergei Skripal, 66, and his daughter Yulia, 33, are found unconscious on a park bench in Salisbury.
– March 7
Police say a nerve agent was used to poison the pair and the case is being treated as attempted murder.
– March 8
Then home secretary Amber Rudd says a Wiltshire Police officer, Detective Sergeant Nick Bailey, is seriously ill in hospital.
– March 12
Prime Minister Mrs May tells the House of Commons the nerve agent is of Russian origin and the Government has concluded it is “highly likely” Russia is responsible for the poisoning.
– March 14
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”.
– March 15
Leaders of Britain, the US, Germany and France issue a joint statement blaming Russia for the attack.
– March 17
Russia announces the expulsion of 23 UK diplomats and says it will shut down the British Council and British Consulate in St Petersburg.
– March 22
DS Bailey is discharged from hospital but says life will “probably never be the same”.
– March 26
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”.
– March 28
Scotland Yard reveals Mr Skripal and his daughter first came into contact with the nerve agent at his home.
– April 3
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.
– April 10
Salisbury District Hospital announces that Ms Skripal has been discharged.
– April 17
The Department for Environment, Food and Rural Affairs says the Novichok used to attack the Skripals was delivered in a “liquid form”.
– May 18
It is announced that Mr Skripal has been discharged from hospital after more than two months of treatment.
– May 26
Businesses in the Maltings area of Salisbury reopen following the attack.
– June 30
Dawn Sturgess and Charlie Rowley fall ill at a flat in Muggleton Road in Amesbury, eight miles from Salisbury, and are taken to hospital.
– July 2
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.
– July 4
Police declare a “major incident” after revealing Ms Sturgess and Mr Rowley have been exposed to an “unknown substance”, later confirmed to be Novichok.
– July 5
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.
– July 6
Forensic investigators in hazardous material suits and gas masks begin searching the building where Ms Sturgess lives.
– July 8
Ms Sturgess dies in hospital after being exposed to Novichok.
– July 9
Scotland Yard launch a murder investigation over her death.
– July 10
Mr Rowley regains consciousness.
– July 13
Police reveal the Novichok that poisoned them was from a small bottle found in Mr Rowley’s home.
– July 20
Mr Rowley leaves hospital, telling reporters he feels lucky to be alive.
– July 30
Ms Sturgess’s funeral service takes place at Salisbury Crematorium.
– August 9
Russia denounces the imposition of “draconian” new US sanctions after the administration concluded Moscow was responsible for the Salisbury attack.
– September 4
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.
– September 5
Scotland Yard and the Crown Prosecution Service say there is sufficient evidence to charge two Russians, Alexander Petrov and Ruslan Boshirov, with offences including conspiracy to murder over the attack. Mrs May reveals that the UK believes they are agents from the GRU military intelligence service.
– September 12
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”.
– September 13
Petrov and Boshirov are interviewed by Russian state-funded news channel RT in which they claim they were tourists visiting the “wonderful” Wiltshire city of Salisbury.
– September 26
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.
– November 5
Zizzi reopens its Salisbury restaurant, where the Skripals dined hours before being found unconscious.
– January 15
DS Bailey returns to active duty.
– January 21
The European Union imposes sanctions including travel bans and asset freezes on Russians blamed for the attack.
– January 24
Mr Rowley’s former home is declared safe.
– March 1
The Ministry of Defence announces Salisbury is to be declared decontaminated of Novichok after an almost year-long military clean-up of 12 sites.
– March 4
A year on from the attack, it emerges intelligence services investigated “increased” and “unusual” activity at the Russian embassy in London in the days before and after the Novichok poisoning.
– June 28
Mrs May and Mr Putin meet in the margins of the G20 summit in Osaka, with the Prime Minister pushing for the two suspects to face justice.
Ahead of the talks the Prime Minister said: “I’m going to make absolutely clear the position the UK takes in relation to what happened in Salisbury.”