There is "no alternative conclusion" than to believe Russia is "culpable" for the attempted murder of a former spy and his daughter with a nerve agent, the UK prime minister says.
Sergei Skripal, 66, and Yulia Skripal, 33, remain in a critical but stable condition after being found slumped on a bench near the Maltings shopping centre in Salisbury, Wiltshire, on Sunday 4 March.
A police officer who fell ill tending to the pair remains seriously ill in hospital, while another person is being treated as an outpatient.
Theresa May said the chemical used in the attack had been identified as being part of a group of nerve agents developed by Russia known as Novichok.
Russia failed to meet Tuesday's deadline to provide a credible explanation over the origins of the substance and Mrs May has invoked several sanctions to send a "clear message".
Timeline of events
- Yulia Skripal flew into London's Heathrow Airport on a flight from Russia at about 14:40 GMT on 3 March
- On 4 March, at about 13:40 GMT, Mr Skripal and his daughter arrived at the Sainbury's upper level car park in Salisbury city centre
- Police said the pair went to the Bishop's Mill pub before going to Zizzi restaurant at 14:20 GMT, staying until 15:35 GMT
- At 16:15 GMT emergency services received the first report of an incident
- Police found the pair on a bench outside Zizzi restaurant in an "extremely serious condition"
- A police officer who fell ill after attending the incident - Det Sgt Nick Bailey - was also taken to hospital and remains in a serious condition
- Of the 38 people who have been seen in hospital in relation to the incident, 34 have been discharged
- Only Mr Skripal, his daughter Yulia and Det Sgt Bailey remain in hospital. One person is being monitored as an outpatient but is not showing any signs of exposure
The investigation so far
Police have been treating the case as attempted murder.
Traces of the nerve agent were found at the Mill pub and Zizzi restaurant, where the Skripals spent the afternoon.
Eyewitness Jamie Paine said he saw a woman on a bench frothing at the mouth and her eyes "were wide open but completely white".
A doctor, who was shopping with her husband in the city centre on Sunday, said Ms Skripal was "slumped in her seat, completely unconscious" and had lost control of her bodily functions.
Assistant Commissioner Mark Rowley, head of counter-terrorism operations, said the Skripals had been "targeted specifically".
Up to 500 people who visited the pub or the restaurant on Sunday or Monday were told to wash their clothes and possessions to avoid any contamination.
Prof Dame Sally Davies, the chief medical officer for England, stressed the risk of harm was "low" but there was some concern that prolonged exposure over weeks and months could cause health problems.
Metropolitan Police assistant commissioner Neil Basu said the "prime focus" of the investigation, which will take many weeks, was how the poison was administered.
Police have appealed for any witnesses who saw the pair in Mr Skripal's red BMW - licence plate number HD09 WAO - between 13:00 GMT and 13:45 GMT on the day of the attack.
Officers have identified more than 240 witnesses and are looking at over 380 pieces of evidence.
More than 250 counter-terrorism officers are involved in the investigation, and about 180 military personnel were deployed to help remove vehicles and objects which may have been contaminated.
Personnel from the Defence Chemical Biological Radiological and Nuclear Centre at Porton Down in Wiltshire identified the nerve agent.
Areas of Gillingham in Dorset have also been cordoned off as part of the investigation.
About 20 miles from the town a truck, which is thought to have recovered Mr Skripal's car from Salisbury, has been sealed off by police and the Army.
How has the Britsh government responded?
The government has requested answers from the Russian leadership but explanations have not been forthcoming.
Mrs May said Russia has responded to the situation with "sarcasm, contempt and defiance" and not provided any credible reasons.
During Prime Minister's Questions, she told MPs the only conclusion to be drawn was one where Russia was "culpable" for the attempted murders and threatening the public safety of Britons in Salisbury.
She added there would be a "full and robust" response beyond what had been done for the Litvinenko poisoning case.
A series of sanctions have been announced including:
- The expulsion of 23 diplomats - who must be gone within a week
- Ministers and the royal family will not attend the Fifa World Cup in Russia later this year
- Russian state assets will be frozen if there is evidence they will be used as a weapon against UK nationals and residents
- Checks on private flights, customs and freight will be increased
- All planned high-level contacts between the UK and Russia will be suspended
- The retraction of the state invitation to Russian's Foreign Minister Sergei Lavrov
Mrs May added there were other measures ready to be deployed if the UK faces more "Russia provocation".
She has already spoken to allies US President Trump, German Chancellor Angela Merkel and French President Emmanuel Macron, who have all agreed to "co-operate closely".
The UK is to brief the UN Security Council on the investigation on Wednesday evening.
What has Russia said?
Russia has dismissed suggestions linking Moscow to the incident.
The Russian foreign ministry said the "hostile" measures were "an unprecedentedly crude provocation" which undermined the relationship between the two countries.
The UK has been "non-transparent" in their investigations and sanctions against Britain are up for discussion, it added.
Foreign minister Sergei Lavrov has said the UK is "playing politics" and not taking into account an international pact on chemical weapons.
He said if the UK sends Moscow a formal request for an explanation under the Chemical Weapons Convention, Russia will respond within the set 10-day time limit.
Russia has also requested to be given a sample of the nerve agent used.
The Russian Embassy in Britain has described the order for diplomats to leave as "unacceptable, unjustified and short-sighted".
Meanwhile a suspect in the 2006 murder of former FSB officer Alexander Litvinenko has told Russian news agency Interfax that determining responsibility has to be done by "serious expert analysis".
Andrei Lugovoi said: "Any chemist or physicist will tell you that in order to determine the involvement or non-involvement of a country, there must at least be some serious expert analyses carried out at a serious expert level.
"When such statements are made within a few days (of the incident), the only thing this shows is the irresponsibility of the person who makes them. It may also indicate that to find the truth is not the aim."
Who are the victims?
Colonel Skripal is a retired Russian military intelligence officer who was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK's Secret Intelligence Service, MI6.
He was jailed for 13 years by Russia in 2006.
In July 2010, he was one of four prisoners released by Moscow in exchange for 10 Russian spies arrested by the FBI. He was later flown to the UK.
According to BBC Newsnight's diplomatic editor Mark Urban, in recent years Mr Skripal gave lectures at military academies offering insights into Russia's foreign military intelligence agency, the GRU.
A friend from college, Vladimir Svyatski, described Mr Skripal as "very active, with a positive attitude and creative".
A former colleague, Oleg Ivanov, told the BBC he was "the life and soul of party"
His daughter, Yulia, would regularly travel to the UK from her home in Moscow to visit her father, relatives told the BBC.
"She told me she liked everything [in the UK]," childhood friend Irina Petrova said. "They had an amazing place, and amazing house."
She had an "excellent" relationship with her father, Ms Petrova said, and had been the "perfect kid", getting excellent grades at school.
Ms Skripal, who friends say worked for multinationals including Nike and PepsiCo, was "always smiling, just like her mother", Ms Petrova added.
What else do we know about the family?
Mr Skripal's wife, Liudmila, died in 2012 after suffering from cancer. His elder brother and son have died in the past two years.
Some of the deaths, the family believe, were in suspicious circumstances.
His son, Alexander Skripal, died aged 43 last July in St Petersburg from liver failure. He is buried in Salisbury, close to his mother.
Mr Skripal's family deny that he worked for MI6 and believe that the espionage case was fabricated by Russia.
Has this happened in the UK before?
The possibility of an unknown substance being involved has drawn comparisons with the 2006 poisoning of Alexander Litvinenko.
The former Russian intelligence officer died in London after drinking tea laced with a radioactive substance.
A public inquiry concluded that his killing had probably been carried out with the approval of the Russian President, Vladimir Putin.
An investigation by Buzzfeed News claims that there have been at least 14 deaths in the UK that US officials suspected were connected to Russia.