The British Home Secretary has described the nerve agent attack on a former Russian spy and his daughter as 'brazen and reckless'.
Sergei Skripal (66) and his daughter Yulia (33) remain in a critical condition in hospital in Salisbury after they were found unconscious on a bench outside a shopping centre on Sunday.
Police yesterday confirmed that the pair had been exposed to a nerve agent, although details of the chemical used have not yet been revealed.
The attack is being treated as a "major incident involving attempted murder".
A police officer who initially responded to the incident is also said to be in a serious condition in hospital.
The officer in question has been named as Detective Sergeant Nick Bailey of Wiltshire Police.
Acting chief constable of Wiltshire Police, Kier Pritchard, told Sky News that a total of 21 people have been treated in the wake of the incident, although only three people - Sergei Skripal, Yulia Skripal and Nick Bailey - remain in hospital.
Earlier, British Home Secretary Amber Rudd gave an update on what she called a "most outrageous crime", saying the investigation is moving "at pace".
She told MPs: "The officer remains serious but stable, and is conscious, talking and engaging. Officer from Wiltshire are providing support to the officer's family and colleagues.
"I can confirm that it is highly likely the police officer has been exposed to the same nerve agent."
Speaking about the attack itself, she observed: "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.
"If we are to be rigorous in this investigation, we must avoid speculation and allow police carry on their investigation [...] We will respond in a robust and appropriate manner once we ascertain who was responsible."
The investigation is being led by counter-terrorism specialists from London's Met Police.
Sergei Skripal, a former Russian military intelligence officer, was convicted in Russia in 2006 of passing state secrets to Britain.
He was later given refuge in the UK as part of a spy swap.