Update - 4.54pm: The police officer who was left seriously ill after assisting poisoned ex-Russian spy Sergei Skripal and his daughter Yulia has been named by sources as Sergeant Nick Bailey.
Temporary chief constable of Wiltshire Police Kier Pritchard said: "I did go and see Nick today and I met Nick and his wife at the hospital in the intensive care unit.
"I've known Nick for many years, he's a great character, he's a huge presence in Wiltshire Police - well liked, well loved, a massively dedicated officer. He's clearly receiving high specialist treatment.
"He's well, he's sat up. He is not the Nick that I know but of course he's receiving a high level of treatment. He's in the safe hands of the medical professionals working in Salisbury District so I'm very confident he's getting the best professional support that he can.
"Of course he's very anxious, he's very concerned. He did his very best on that night.
"All of our staff that attended the incident in Salisbury in the Maltings, they performed the role that police officers and police staff do every day up and down the country. Limited information, responded to try and protect people and safeguard people who we knew were ill.
"I'm massively proud of what Nick did and all of my staff on that night, they did a first-class job."
Wiltshire Police said in a statement: "We can confirm the Wiltshire officer currently in Salisbury District Hospital is Detective Sergeant Nick Bailey.
"Nick joined Wiltshire Police in 2002.
"Temporary Chief Constable Kier Pritchard visited Nick in hospital today along with his wife. He remains in a serious but stable condition.
"A photo of Nick is attached.
"His family has asked for privacy at this time and do not want to speak directly with the media."
In tweets today, the Russian Embassy said: "Totally agree with Secretary @AmberRuddHR: first evidence then conclusions on Mr Skripal's case. Responsible political approach."
Totally agree with Secretary @AmberRuddHR: first evidence then conclusions on Mr Skripal's case. Responsible political approach. pic.twitter.com/2mUnoVnUd6— Russian Embassy, UK (@RussianEmbassy) March 8, 2018
And quoting the UK's Ministry of Foreign Affairs, referring to two other high-profile deaths of Russians in the UK, it said: "MFA: when Boris Berezovsky and Alexander Perepilichny died in Britain, there was a lot of speculation in the media, then all the conclusions were classified, and no data provided to Russia. Same happening now, with MI6 agent Sergei Skripal poisoning."
12.19pm: UK condemns 'brazen and reckless' use of nerve agent against Russian ex-spyBritish Home Secretary Amber Rudd has told the British House of Commons that the use of a nerve agent in British is "a brazen and reckless act".
In her statement to the Commons, she said Mr Skripal and his daughter Yulia "remain unconscious and in a critical but stable condition".
She added: "I regret to inform the House that a police officer has also fallen seriously ill. The officer was one of the first responders on Sunday, acting selflessly to help others.
"The latest update from the hospital is that the officer remains serious but stable and is conscious, talking and engaging.
"Our thoughts are with all three victims, their families and friends at what for them will be an incredibly difficult time."
Ms Rudd said she would not comment further on the nature of the nerve agent as it is a "fast-paced criminal investigation".
Addressing speculation on who was responsible for "this most outrageous crime", Ms Rudd added: "The use of a nerve agent on UK soil is a brazen and reckless act. This was attempted murder in the most cruel and public way. People are right to want to know who to hold to account.
"But if we are to be rigorous in this investigation, we must avoid speculation and allow the police to carry on their investigation.
"We are committed to doing all we can to bring the perpetrators to justice, whoever they are and wherever they may be.
"The investigation is moving at pace and this Government will act without hesitation as the facts become clearer."
11.03am: Officer 'talking in hospital' as police probe murder bid of Russian ex-spy by nerve agent
The police officer who rushed to the aid of a Russian ex-spy targeted with a nerve agent in England is talking in his hospital bed as detectives race to discover who was responsible.
British Home Secretary Amber Rudd said the policeman was not in intensive care but was still in a serious condition following the attack, in which a nerve agent, described by Ms Rudd as "very rare", was used in an attempt on the lives Sergei Skripal, 66, and 33-year-old daughter Yulia.
Counter-terror police are working to unravel what is now feared to be a sophisticated chemical weapon plot amid heightened tensions between Britain and the Kremlin.
UK Defence Secretary Gavin Williamson said Russia was becoming an "ever-greater threat", while a senior general warned the state was "capable of anything".
Ms Rudd refused to say whether she regarded Russia as responsible, saying the investigation should be based on "facts, not rumour".
And speaking about the officer, she told BBC Radio 4's Today programme: "I heard this morning from the head of counter-terrorism policing that he is engaging with, talking to, people.
"But that doesn't mean his situation isn't serious. It remains serious. He is not in intensive care, but it's a serious situation."
Mr Skripal and his daughter remain critically ill.
Ms Rudd is expected to make a statement to the House of Commons about the incident shortly after noon today.
7.49am: UK police investigating nerve agent attack against Russian ex-spy
Counter-terror police are working to unravel what is now feared to be a sophisticated chemical weapon plot targeting a Russian spy and his daughter.
A nerve agent is believed to have been used to critically injure Sergei Skripal, 66, and 33-year-old Yulia in Salisbury, Wiltshire, on Sunday.
One of the first police officers to arrive at the bench where the pair were slumped is also seriously ill in hospital.
It remains unclear who is responsible for poisoning the pair, but the attack has stoked tensions between Britain and Russia amid suspicions of state responsibility.
Former British ambassador to Russia Sir Andrew Wood told the Daily Telegraph that the "assassination attempt" was more serious given a policeman was among the injured.
However the former diplomat, who served in Moscow between 1995 and 2000, said the injuries suffered by the double agent's daughter and the officer should not take attention away from the attempt on Mr Skripal's life.
He told the paper: "If it is true that this is, in some fashion, the Russian state, it obviously makes it even harder to believe the Russian state is worth anything or is to be trusted.
"The fact they targeted his daughter, and that a policeman is seriously ill, makes it emotionally difficult, but it does not alter the fact that this was an attempted assassination on British soil."
Home Secretary Amber Rudd is expected to make a statement to the House of Commons about the incident on Thursday.
Assistant Commissioner Mark Rowley, the head of counter-terrorism policing, revealed on Wednesday that the incident was being treated as attempted murder and the pair had been "targeted specifically".
He declined to specify the nerve agent or how it was administered.
He said: "Having established that a nerve agent was the cause of the symptoms, leading us to treat this as attempted murder, I can also confirm that we believe the two people who originally became unwell were targeted specifically.
"Our role now of course is to establish who is behind this and why they carried out this act."
Hundreds of detectives, forensic officers and analysts are working on the case, which has drawn comparisons to the poisoning of former Russian intelligence officer Alexander Litvinenko on British soil in 2006.
Nerve agents, which are chemical weapons, have been used in assassinations and attacks in war zones in recent years.
Kim Jong Un's half-brother Kim Jong Nam was killed at an international airport in Malaysia last year in an attack using a nerve agent known as VX.
Another well-known nerve agent, sarin gas, killed more than 90 people in a rebel-held area in Khan Sheikhoun, Syria, drawing international condemnation of the Bashar Assad regime.
Access to such toxins are tightly regulated, meaning the Salisbury plot would have taken considerable planning to execute.
Hamish de Bretton-Gordon, a former commanding officer of Britain's Joint Chemical, Biological, Radiological and Nuclear Regiment, told the Times: "This is not the stuff you can knock up in your back shed.
"It is quite challenging to make. The inference is that this has probably come from a major laboratory, probably state-run."
Mr Rowley reiterated his appeal for anyone who was in Salisbury city centre on Sunday to come forward to help with the "missing pieces" in the case.
Dame Sally Davies, the chief medical officer, said the incident posed a "low risk" to the public and advised that all the sites the pair were known to have visited had been "secured".
Public Health England later confirmed it had contacted all first responders who had attended the scene.
Russia have denied responsibility for the attack, which comes seven years after Mr Skripal was released from the country as part of a spy swap with the US.
He had been convicted in his home country in 2006 for passing state secrets to MI6.
The investigation has triggered a diplomatic row and prompted crisis talks in Whitehall but Home Secretary Amber Rudd said police must respond to "evidence, not to rumour".
It comes as police extended the cordons in Salisbury city centre, and also sealed off part of a business park in nearby Amesbury.
The Defence Science and Technology Laboratory in nearby Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is believed to have been involved in examining the substance.
- Press Association