Counter-terror police are working to unravel what is now feared to be a sophisticated chemical weapons plot targeting a Russian spy and his daughter.
A nerve agent is believed to have been used to critically injure 66-year-old Sergei Skripal and 33-year-old Yulia in Salisbury, Wiltshire, on Sunday (March 4).
One of the first police officers to arrive at the bench where the pair were slumped was also left seriously ill in hospital, but his condition is said to be improving.
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.
Sir Andrew Wood, a former British ambassador to Russia, 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 Skripal's daughter and the officer should not take attention away from the attempt on the former spy'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 today (March 8).
Assistant Commissioner Mark Rowley, the head of counter-terrorism policing, revealed yesterday (March 7) that the incident was being treated as attempted murder and the pair had been "targeted specifically."
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.
Russia has denied responsibility for the attack, which comes seven years after Skripal was released from the country as part of a spy swap with the US.