Update - 5.15pm: British Prime Minister Theresa May has told the UK's House of Commons that it is "highly likely" that Russia is responsible for the nerve agent attack on Sergei Skripal and his daughter Yulia.
Mrs May has told the House of Commons that analysis of the nerve agent used in the attack on Sergei Skripal and his daughter in Salisbury shows that it was from a group known as novichok.
Making an oral statement on the latest information about the case to the Commons following a meeting of the British Government's National Security Council, Mrs May told MPs: "It is now clear that Mr Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia.
"This is part of a group of nerve agents known as novichok.
"Based on the positive identification of this chemical agent by world-leading experts at the Defence, Science and Technology Laboratory at Porton Down, our knowledge that Russia has previously produced this agent and would still be capable of doing so, Russia's record of conducting state-sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassinations, the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal."
The British Prime Minister said the officer who attended the scene of the "reckless" attack in Salisbury is in a serious but stable condition.
Mrs May said: "There are only two plausible explanations for what happened in Salisbury on March 4.
"Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others."
Mrs May said that the UK's Foreign Secretary Boris Johnson summoned the Russian ambassador to the Foreign Office on Monday afternoon and asked him "to explain which of these two possibilities it is, and therefore to account for how this Russian-produced nerve agent could have been deployed in Salisbury against Mr Skripal and his daughter".
Mr Johnson told the ambassador, Alexander Yakovenko, that Moscow must "immediately provide full and complete disclosure of the novichok programme to the Organisation for the Prohibition of Chemical Weapons", said Mrs May.
The Foreign Secretary requested a response from the Russian Government by the end of Tuesday.
3.27pm: Russian embassy accuses UK of playing 'very dangerous game' over Sergei Skripal attackA Russian embassy spokesman accused the UK Government of playing a "very dangerous game" with British public opinion and warned of the risk of "serious long-term consequences".
In a statement on the embassy website, a spokesman said: "We would like to stress once again that we are outraged by the anti-Russian media campaign, condoned by the Government, that influences the investigation and has a psychological effect on British residents.
"Our compatriots and British nationals of Russian origin are worried about their future in this country. UK-based Russian journalists are receiving threats.
"Current policy of the UK Government towards Russia is a very dangerous game played with the British public opinion, which not only sends the investigation upon an unhelpful political track but also bears the risk of more serious long-term consequences for our relations."
The spokesman said that the day after the first reports of the poisoning emerged, the embassy had requested information from the Foreign Office about "their health situation and the investigation".
The spokesman said British officials "don't provide any additional information and don't distance themselves from the media campaign".
Russian President Vladimir Putin dismissed questions about his state's alleged involvement in the Skripal case.
On a visit to a grain centre, he told the BBC: "We're dealing with agriculture here ... and you talk to me about some tragedies.
"Get to the bottom of things there, then we'll discuss this."
12.47pm: Here are some options for Theresa May as she prepares to make a statement on the Sergei Skripal attack
British Prime Minister Theresa May will make a statement to update MPs on the nerve agent attack on a Russian spy Sergei Skripal and his daughter Yulia at around 4.30pm.
Ahead of her statement, Downing Street said "if we get to a position when we are able to attribute this attack then we will do so".
The British Government will be under pressure to take a tough line against Russia if it is proven to be involved in the Salisbury nerve agent attack.
The expulsion of Russian diplomats and spies, new financial sanctions against individuals linked to the Kremlin, and withdrawal of official representation from this summer's World Cup are among the retaliatory measures which could be on the table if Britain takes action.
Here are some of the options which may be considered:
- World Cup snub
Official representation could be withdrawn from the World Cup in Russia if Kremlin links are proven.
Prime Minister Theresa May told MPs last week that the Government would "look at whether ministers and other dignitaries should attend" the tournament in that event.
The Duke of Cambridge has already said he has no plans to attend.
- World Cup boycott
There have been calls for the England team to boycott the tournament but their withdrawal would probably have little impact.
A coordinated protest, involving the withdrawal of multiple countries, would be far more effective in damaging Vladimir Putin's showcase international event and sending a strong message to Russia.
However, experts say the nerve agent poisoning in the small market town of Salisbury is unlikely to create enough international momentum to trigger a wider boycott.
- Statement of condemnation
Britain may call on the support of its closest allies if there is evidence of a Russian murder attempt on UK soil.
A joint statement of international condemnation could be issued from leaders including French President Emmanuel Macron and Germany's Angela Merkel, warning Russia that such actions will not be tolerated.
- Expulsion of Russians
Senior Russian diplomats and spies could be immediately expelled from the UK in retaliation.
The murder of former spy Alexander Litvinenko in 2006 led to a severe deterioration in British-Russian ties.
In July the following year, four British and four Russian diplomats were expelled from their respective embassies.
- Financial sanctions
Individuals linked to the Kremlin may be subject to financial sanctions including the withdrawal of visas and seizing of assets.
A series of sanctions were imposed by the European Union and United States after the downing of Malaysia Airlines flight MH17 in Ukraine in 2014, targeting access to capital markets, the oil and gas sectors and individuals.
The EU could tighten these measures, which it has not kept current, in the wake of the Salisbury attack to increase pressure on Russia.
11.53am: Investigators studying high-quality CCTV footage of Sergei SkripalHigh-quality CCTV footage of Sergei Skripal and his daughter in the moments before they collapsed has been passed to investigators probing the attack of a Russian former double agent.
The images are understood to be clear enough to see people's faces or read a car number plate.
There were fears the city's £400,000 CCTV system may not have been working after it was besieged by technical problems.
But Matthew Dean, leader of Salisbury City Council, said the issues had been resolved around a month ago.
Crucially, the coverage includes The Maltings area, where Mr Skripal, 66, and Yulia, 33, were found collapsed on a bench eight days ago.
"The CCTV system was fully functional and a great deal of footage has been shared with the enquiry," he said.
"I can confirm some very high quality footage was shared on the Monday with the enquiry."
The images showing the victims in the hours and minutes before their collapse could be invaluable to investigators, who continue to scour the city for clues.
Police cordons remain in place at various locations in Salisbury, while the military have been working with counter-terror police to remove potentially contaminated vehicles and other items.
"People are used to seeing the military around," said Mr Dean.
"I think we are very fortunate as we have a military presence anyway.
"It is the home of the British Army and Porton Down, the chemical weapons research facility, is located just outside of the city."
The Conservative politician also defended fresh health advice issued a week after the attack when it emerged traces of a nerve agent had been found at The Mill pub and the nearby Zizzi restaurant.
"If there is anger I haven't discerned it. People accept that this incident is unprecedented in the British Isles and it took Public Health England some days of investigative science to find out even what the cause of injury was to Mr Skripal and his daughter.
"So the advice of yesterday was following the last few days of scientific analysis."
10.14am: UK Foreign Affairs chief: Nerve agent attack 'looking like it was state-sponsored attempted murder'
The attack on Russian spy Sergei Skripal and his daughter Yulia is "looking awfully like it was state-sponsored attempted murder", the chairman of the UK's Foreign Affairs committee has said.
Tom Tugendhat said he would be surprised if Theresa May, who is chairing crunch talks on the Salisbury nerve agent attack, did not blame the Kremlin.
He told BBC Radio 4's Today programme: "It's a bit too early to be absolutely certain of that but we are expecting to see the Prime Minister make an announcement soon.
"And, frankly, I would be surprised if she did not point the finger at the Kremlin."
Mr Tugendhat warned that English football fans travelling to Russia for the World Cup may be at risk of harm if tensions escalate between London and Moscow.
He said: "We do need to be very, very careful for British fans who are travelling there that they are not in any way caught up in the politics of this.
"And, I'm afraid the danger of Russia responding to British fans for actions taken by their government is all too real."
In response to a request for comment from the Press Association, Vladimir Putin's press secretary Dmitry Peskov said: "We consider inappropriate any mention of the Russian government in the context of what happened to Sergei Skripal.
"We have nothing to do with the story."
9.18am: Theresa May to chair talks on Salisbury nerve agent attack
Theresa May is to chair crunch talks on the Salisbury nerve agent attack amid speculation the British Government is moving closer to publicly blaming Russia.
The British Prime Minister has summoned a meeting of the National Security Council (NSC) where ministers will be briefed on the latest intelligence on the incident.
She is likely to come under intense pressure to take tough action if a clear link to the Kremlin has been established.
The meeting comes as former Russian double agent Sergei Skripal and his daughter, Yulia, remain critically ill in hospital eight days after they were found collapsed on a bench in the Wiltshire city.
Detective Sergeant Nick Bailey is in a serious but stable condition in the Salisbury District Hospital, where he is said to be conscious and talking.
The NSC brings together senior ministers from relevant departments and is attended by intelligence and military chiefs as required.
Measures potentially on the table if they do decide to take action could include the expulsion of Russian diplomats and spies, new financial sanctions against individuals linked to the Kremlin, and withdrawal of official representation from the World Cup in Russia.
On Sunday, hundreds of diners and pub-goers were urged to wash clothes and other items a week after potentially coming into contact with the nerve agent.
The "precautionary advice" was issued after traces of the substance were found in The Mill pub and the nearby Zizzi restaurant, in Salisbury.
In other developments Wiltshire Police told residents "not to be alarmed" as counter-terror police were helped by Armed forces, including Royal Marines, in the clean-up operation as:
Salisbury Spy: Former KGB agent says police have not been in touch about threats he claims he has received. He believes that Russian services are behind the Wiltshire attack and others abroad. Russia denies any involvement. pic.twitter.com/x9YlkCw4oI— Good Morning Britain (@GMB) March 12, 2018
England's chief medical officer Sally Davies said she believed up to 500 people would have been in either of the two venues between the Sunday lunchtime and Monday night.
She said: "I want to reassure the general public that the risk to us all from this incident in Salisbury has not changed, and that the risk to us all remains low."
The advice included machine wash clothing worn on the day, and double-bagging items which would normally be dry cleaned to await further advice.
Jamie Knight, 30, from Salisbury, will appear before magistrates in Swindon on Monday accused of breaching one of the cordons on Friday evening.