Why is the UK accusing Russia of launching a nerve agent attack on Sergei Skripal in Salisbury, and what is the evidence?

Military personnel wearing protective suits remove a police car and other vehicles from a public car park as they continue investigations into the poisoning of Sergei Skripal on 11 March in Salisbury (Getty)

“There is no alternative conclusion other than that the Russian state was culpable for the attempted murder of Sergei Skripal and his daughter - and for threatening the lives of other British citizens.”

That was the conclusion Theresa May delivered to MPs over the nerve agent attack in Salisbury, sparking the expulsion of 23 Russian diplomats and a wave of reprisals that have been met with an angry response from the Kremlin.

Russian officials have denied involvement, branded the Prime Minister’s statement a “circus show” and accused the UK of violating a chemical weapons convention by refusing to send it samples of the substance used.

Meanwhile, conspiracy theories are swirling online, with proponents blaming a range of targets including the US, Ukraine, the EU and Britain itself for the attack.

Police have not yet confirmed how Mr Skripal and his daughter Yulia were exposed to the substance, which left them in a critical condition in hospital and affected 36 other people.

1. What do we know about the attack?

   Ms Skripal, 33, was visiting her father and arrived in Heathrow Airport on a flight from Russia at on 3 March.

   At approximately 1.40pm the following day, the pair arrived in Sainsbury’s upper level car park in The Maltings shopping precinct in Salisbury.

   They went to The Mill pub before going to Zizzi’s Italian restaurant at 2.20pm, where they stayed until around 3.35pm.

   Emergency services were called for the first time 40 minutes later and found Mr Skripal and his daughter “extremely ill” on a nearby bench.

   Investigations by military experts at the Defence, Science and Technology Laboratory at Porton Down identified the substance used as from a group of nerve agents known as “Novichoks”, roughly translating as “newcomer” in Russian.

2. What are Novichoks?

   The name was given to a group of nerve agents developed by the Soviet Union in a covert programme that was revealed by defectors.

   Vil Mirzayanov, a chemical weapons developer who fled to the US, said Novichoks were up to 10 times more deadly than VX.

   He told Reuters the programme involved as many as 30,000 or 40,000 people, including around 1,000 who worked on Novichoks specifically, although many were not aware of the initiative’s true nature.

   Dr Mirzayanov said they could be synthesised by mixing harmless compounds together, making it made it easier for Russia to produce the necessary materials under the cover of manufacturing agricultural chemicals.

   He described the effects of the chemical including extreme pain and suffocation, as it paralyses the respiratory system, after testing its effects on lab animals.

   “Novichok was invented and studied and experimented and many tons were produced only in Russia - nobody knew in this world,” Dr Mirzayanov said, arguing that only the Kremlin would have the capability to deploy the agent.

   He claimed that Russia maintained tight control over its Novichok stockpiles and that the agent was too complicated for a non-state actor to have weaponised.

   “The Kremlin all the time, like all criminals, denying - it doesn’t mean anything.”

3. How does the Government know Novichok was used in Salisbury?

   The Prime Minister said the nerve agent was identified by military scientists at Porton Down, which formerly developed nerve agents including VX itself.

   No details of the chemical analysis have been provided publicly but samples are being sent to the Organisation for the Prohibition of Chemical Weapons (OPCW) for verification.

   Dr Patricia Lewis, research director for International Security at Chatham House, said Novichoks could be identified because they have distinct chemical formulae.

   “There could be contaminants that would give away where it has come from,” she told The Independent, adding that high-resolution trace analysis could detect pollen and other clues.

   There are also different processes for manufacturing the chemical precursors of Novichoks that can be used to establish their origin or glean additional information.

   Unlike other most other nerve agents, which are most commonly administered as liquids, the group can also be used in powder form.

4. Why has the British Government named Russia as the culprit?

   Briefing the UN Security Council on Wednesday, British ambassador Jonathan Allen said Novichok was “not a weapon which can be manufactured by non-state actors”.

   “It is so dangerous that it requires the highest-grade state laboratories and expertise,” he added.

   “Based on the knowledge that Russia has previously produced this agent and combined with Russia’s record of conducting state sponsored assassinations – including against former intelligence officers whom they regard as legitimate targets – the UK Government concluded that it was highly likely that Russia was responsible for this reckless act.”

   The Prime Minister gave Russia 24 hours to provide an explanation of how Novichok was used on the streets of Wiltshire, initially raising the possibility that Vladimir Putin’s government might have “lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others”.

   But the Russian government refused the demand, instead calling on Britain to hand over a sample of the nerve agent and accusing it of unacceptable “provocations”.

   “They have provided no credible explanation that could suggest they lost control of their nerve agent,” Ms May told MPs on Wednesday.

   “No explanation as to how this agent came to be used in the United Kingdom; no explanation as to why Russia has an undeclared chemical weapons programme in contravention of international law.

   “Instead they have treated the use of a military grade nerve agent in Europe with sarcasm, contempt and defiance.

   “There is no alternative conclusion other than that the Russian state was culpable for the attempted murder of Mr Skripal and his daughter - and for threatening the lives of other British citizens in Salisbury.”

   Sceptics have questioned why the Russian government would seek to murder Mr Skripal after previously jailing him for treason and agreeing to hand him over to the West in a major Cold War-style spy swap in 2010.

   They claim the Kremlin would not seek to carry out such a brutal and obvious assassination attempt ahead of presidential elections and during tensions over alleged election interference and the wars in Ukraine and Syria, while it would also endanger future spy swaps.

   But those supporting the British Government's conclusion say the attack could be a show of power from Mr Putin, a "declaration of indifference to the suspicions of others" that intimidates allies and enemies alike.

   Mr Skripal was viewed as a traitor after feeding state secrets to MI6 during his time as a GRU military intelligence officer, and may have been "freelance" spying for private intelligence firms in the UK.

5. What is Russia saying?

   The Kremlin has denied involvement in the attack and accused the UK of violating the Chemical Weapons Convention.

   But British authorities insist there are no provisions requiring them to share samples collected in the ongoing criminal investigation.

   “It is therefore Russia which is failing to comply with the provisions of the convention and the UN Security Council should not fall for their attempts to muddy the waters,” Mr Allen said.

   Russian politicians have contradicted each other, with some claiming that Novichok stockpiles were destroyed and others claiming that it was never developed in the first place.

   The state-owned English language Sputnik website quoted Igor Morozov as saying: “Russia has not only stopped the production of nerve agents, including Novichok, but also completely destroyed all their reserves. This was done in accordance with international agreements under the control of OPCW international observers.”

   But deputy foreign minister Sergei Ryabkov told Interfax news agency: “I want to state with all possible certainty that the Soviet Union or Russia had no programmes to develop a toxic agent called Novichok.”

   Vassily Nebenzia, Russia’s representative at the UN Security Council, repeated the assertion and told delegates: “In the Russian Federation there was no scientific research or development work under the title Novichok…in 1992 in the Russian Federation, Soviet developmental work was stopped, and in 2017 it completed the destruction of all existing stocks of chemical weapons. “

   Russia announced the destruction of its declared chemical weapons stockpiles in September, but had never admitted the creation of Novichoks or the development facilities associated with them.

   Dr Lewis said the fact Novichoks were not listed substances on the global Chemical Weapons Convention created a “loophole” meaning they were not monitored by international agencies.

   Russia has said it will cooperate with the OPCW’s investigations, but attacked the organisation just four months ago after it found ally Bashar al-Assad had used sarin in rebel-held areas in Syria.

   Foreign ministry officials called an OPCW report biased, "unprofessional and amateur”, claiming that civilian deaths in Khan Sheikhoun may have been “staged”.

   Russia blocked UN Security Council action against its Syrian allies, then proposed changing the rules for inspectors at the Hague-based watchdog in ways that Western diplomats said would undermine its work.

6. Where else could Novichok have come from?

   Some analysts have claimed that Novichoks could have been smuggled out of chemical weapons and storage sites after the collapse of the Soviet Union, when facilities were abandoned by unpaid staff and security was low.

   Whistleblowers said they were only ever made in Shikhany, central Russia, and then tested at the Nukus site in Uzbekistan, which was decontaminated with US support.

   While weaponised nerve agents degrade over time, if the precursor ingredients were smuggled out in the 1990s, stored in proper conditions and mixed recently, they could still be deadly in a small-scale attack according to some experts.

   “Could somebody have smuggled something out?” said Amy Smithson, a chemical weapons expert at the James Martin Center for Nonproliferation Studies.

   “I certainly wouldn’t rule that possibility out, especially a small amount and particularly in view of how lax the security was at Russian chemical facilities in the early 1990s.”

   In 1995, a Russian banking magnate called Ivan Kivelidi and his secretary died from organ failure after being poisoned with a military grade toxin found on an office telephone.

   A closed trial found that his business partner had obtained the substance via intermediaries from an employee of a state chemical research institute known as GosNIIOKhT, which was involved in the development of Novichoks.

   The employee, Leonard Rink, told police he had been storing poisons in his garage and selling them to pay off debts.

   Novichoks could theoretically be manufactured by other states but experts believe the creation of a weaponised form of the nerve agent outside the former Soviet Union is exceptionally unlikely.

   The binary agents are created by mixing unrestricted chemicals together, but need “extremely specialist facilities with high levels of security and protections for workers”.

   Dr Lewis said that under chemical weapons convention and international law, it would be legal for countries to develop small amounts of Novichoks in order to build defences against them.

   “I would be extremely surprised if since 1992 the chemical weapons experts in a serious number of countries which had chemical weapons capabilities have not sought to make small amounts to defend against it,” she added.

   “It would be legitimate, legal and one would say they have a duty to do so.”

   Dr Lewis said the fact Porton Down scientists identified the substance known as Novichok suggested they had information on its chemical structure.

   There have been mounting concerns about the chemical weapons capability of North Korea, particularly following the murder of Kim Jong-un’s brother using VX in Malaysia last year.

   She said experts were questioning whether the totalitarian state could be manufacturing nerve agents and selling them to non-state actors, but there was no evidence of Novichoks being in North Korea’s arsenal.

7. What are the other theories?

   Some critics have cast doubt on the findings of British intelligence agencies by citing the false conclusions that Saddam Hussain had undeclared weapons of mass destruction, which sparked the Iraq War.

   Jeremy Corbyn suggested that the nerve agent attack could have been carried out by Russian-linked gangsters rather than ordered by Moscow.

   “To rush way ahead of the evidence being gathered by the police, in a fevered parliamentary atmosphere, serves neither justice nor our national security,” he wrote in The Guardian.

   ”In my years in parliament I have seen clear thinking in an international crisis overwhelmed by emotion and hasty judgments too many times.

   “Flawed intelligence and dodgy dossiers led to the calamity of the Iraq invasion.”

   Several alternative theories and conspiracies have been raised, although few provide a credible means and motive for another actor attacking Mr Skripal – a former Russian double agent who was given refuge in the UK after selling secrets to MI6.

   Some have accused Britain itself of launching the attack as a “false flag” to smear Russia, damage Mr Putin ahead of elections or even to distract from a grooming scandal in Telford.

   Mr Nebenzia told the UN Security Council that the “most probable source” of the Novichok was countries that had been carrying out research on nerve agents since the 1990s, including the UK.

   Russian media reports have also suggested that the US stole samples of Novichoks while decontaminating a former Soviet Union testing plant in Uzbekistan from 1999 onwards.

   Proponents of the theory claim that the US may have launched the attack itself, either through the “deep state” or because of Mr Skripal’s potential links to the private security firm that compiled a dossier of allegations against Donald Trump.

   Or, they say, Americans passed the nerve agent or details on how to manufacture it to British allies, with many citing the proximity of Porton Down to the location of the attack.

   Ukraine has been raised as another potential culprit, in the belief that it would seek to discredit Russia to gain international backing against pro-Russian separatists fighting an ongoing war against government forces.

   Other conspiracy theorists have blamed wide-ranging targets including the EU or pro-Remain elements in Britain.

   “I believe this is all to derail Brexit,” said one caller to LBC radio. “It's to make us look isolated and vulnerable against Russia so that we stay in the EU.”

   Ben Nimmo, of the Atlantic Council’s Digital Forensic Research Lab, said many Russian news articles furthering the theories were based on “interviews with former security chiefs and weird commentators” with no expertise or credibility.

   He told The Independent a mounting disinformation campaign was using the “same techniques” as those seen after incidents including the downing of Malaysia Airlines flight 17 using a missile from Russia.

   “You can think of them as dismiss, distort, distract and dismay,” Mr Nimmo said. “You insult the critic, then distort by falsifying the evidence.

   “Distract is the whole ‘if they accuse you, you accuse them’ method, and then there’s the conspiracy theorists. Dismay comes when they threaten ‘horrible things will happen if you do this’.”

   Mr Putin’s spokesperson said Russia “won’t take long” to respond to the UK’s expulsion of diplomats, while a foreign ministry representative suggested British journalists could be kicked out of the country.