Video report by ITV News Correspondent Dan Rivers
Up to 21 people have been treated in connection to the nerve agent attack on a Russian spy and his daughter in Salisbury, the head of Wiltshire police has said.
A number of police officers are among those feared to have been injured, said Kier Pritchard, chief constable for Wiltshire Police.
The numbers include former spy Sergei Skripal and his daughter Yulia, both of whom are fighting for life.
Also included is police Sergeant Nick Bailey, who was among the first on the scene and is in intensive care.
There's been around 21 people [affected]...A number of those have been through the hospital treatment process, they're having blood tests, they're having treatment in terms of support and advice being provided.
Mr Pritchard said an unspecified "smaller" number of the injured were police officers and staff.
The only person still in hospital is Mr Bailey, who remains in intensive care but is conscious and talking.
Sunday's chemical weapon attack is being treated as attempted murder, with hundreds of police officers continuing to investigate who was behind it.
Theresa May has told ITV News that the UK will do "what is appropriate" should evidence prove the agent was state sponsored.
In an exclusive interview with Julie Etchingham, the prime minister warned that "if action needs to be taken the Government will do that", after Sergei Skripal and his daughter Yulia were left critically ill in hospital.
She warned the police need the space and time to conduct their investigation so evidence can be gathered.
"Of course if action needs to be taken then the government will do that," she said. "We'll do that properly, at the right time, and on the basis of the best evidence.
"We will do what is appropriate, we will do what is right, if it is proved to be the case that this is state sponsored. But let's give the police the time and space to actually conduct their investigation."
Mrs May later said her thoughts are with Detective Sergeant Nick Bailey, in a statement paying tribute to the police.
She said: "The events of Sunday are a stark reminder, if ever one was needed, of the dangerous situations our emergency services face and the dedication and courage they display every day in order to keep us safe."
The attack has prompted crisis talks in Whitehall, and has led to a souring of already strained relations between Russia and the UK.
The case is being likened to the murder of ex-Russian spy Alexander Litvinenko, who died in the UK 12 years ago after being exposed to polonium.
A public inquiry concluded in 2016 that the killing had "probably" been carried out with the approval of the Russian president, Vladimir Putin.
While police and government ministers have refused to speculate as to who the perpetrators are, the foreign secretary and defence secretary have made strong statements about Russia's aggressive foreign policy.
Boris Johnson described Russia as a "malign and disruptive force", while Gavin Williamson called Russia "an ever-greater threat".
However backbenchers and opposition MPs were more forthcoming.
Former minister Sir Edward Leigh said it would be "a brazen act of war" if Russia was behind an attack on a Russian ex-spy adding the circumstantial evidence was "very strong" against Russia, and that it was an attempt at "humiliating our country".
Labour former minister Ben Bradshaw also spoke out about the growing threat "of the terrorist Russia state under President Putin".
Home Secretary Amber Rudd told the Commons the attack was "attempted murder in the most cruel and public way" adding: "We must avoid speculation and allow police to continue their investigation."
A Kremlin spokesman described Mr Skripal's illness a "tragic situation", but added that it didn't have "any information".
The Russian Foreign Ministry has accused Western media of using the incident to "incite an anti-Russian campaign".
Skripal, 66, is a former Russian double agent who is believed to have worked for MI6.
He was arrested in Moscow in 2004.
In 2010 he was granted refuge in the UK following one of the biggest spy swaps since the Cold War.
The former spy and his 33-year-old daughter were found slumped on a bench in Salisbury on Sunday.
Later, the police confirmed the pair had been targeted with a nerve agent.
Details around the type of nerve agent used were not disclosed, but investigators are working closely with specialists and scientists.
Police say hundreds of officers are investigating the case.