A nerve agent was used to try to murder a former Russian spy and his daughter, police have said.
Sergei and Yulia Skripal were found unconscious in Salisbury on Sunday afternoon and remain critically ill.
A police officer who was the first to attend the scene is now in a serious condition in hospital, Assistant Commissioner Mark Rowley said.
Nerve agents are highly toxic chemicals that stop the nervous system working and shut down bodily functions.
They normally enter the body through the mouth or nose, but can also be absorbed through the eyes or skin.
Mr Rowley, head of Counter Terrorism Policing, said government scientists had identified the agent used, but would not make that information public at this stage.
“This is being treated as a major incident involving attempted murder, by administration of a nerve agent,” he said.
“Having established that a nerve agent is the cause of the symptoms… I can also confirm that we believe that the two people who became unwell were targeted specifically.”
He said there was no evidence of a widespread health risk to the public.
— Metropolitan Police (@metpoliceuk) March 7, 2018
As a result of the suspected Russian involvement Britain was tonight preparing to expel dozens of Russian diplomats and spies in retaliation for the deadly attack.
Intelligence chiefs and the Foreign Office were drawing up lists of likely candidates to be sent back, on Theresa May’s orders.
The attack – suspected by many to have been ordered by Russian President Vladimir Putin – was “a near-declaration of war” said one source.
And one ex-British intelligence officer said: “This could be the largest expulsion of diplomats and Russian spooks in many years.
“It would be a decisive move to show this is being taken extremely seriously.”
Expulsions would be the first in a possible string of punitive measures once the Government is satisfied the Russian state is responsible for the apparent hit on former Russian spy Sergei Skripal and his daughter.
Some believe the outrage could simply be part of Vladimir Putin’s show of strength in the run-up to the first round of elections he is expected to win on March 18.