The Government is to hold a meeting of its emergency committee Cobra, as a Russian double agent and his daughter continue fighting for their lives in hospital.
They were exposed to a mystery substance which has triggered a diplomatic row and prompted crisis talks in Whitehall.
As the high-profile investigation enters a fourth day, counter-terror officers have extended the cordon yet further after Sergei Skripal was found unconscious in Salisbury, Wiltshire, along with his 33-year-old daughter Yulia shortly after 4pm on Sunday (March 4).
Scotland Yard said detectives were "keeping an open mind as to what happened," and that the incident had not been declared a terrorist incident, adding that it would not "provide a running commentary" on the investigation.
But it has forced the Government to call a meeting of its emergency committee Cobra on this morning (March 7), to be chaired by Home Secretary Amber Rudd.
The cross-departmental meeting, which has previously been held after terror attacks to plot the Government's response to national emergencies, comes as relations between the UK and the Kremlin continue to deteriorate, after Foreign Secretary Boris Johnson went on the offensive in the House of Commons.
Addressing MPs about the "disturbing incident," Johnson noted that this case had "echoes" of the death of Alexander Litvinenko, a Russian dissident who was fatally poisoned in London in 2006.
He said: "While it would be wrong to prejudge the investigation, I can reassure the House that should evidence emerge that implies state responsibility, then Her Majesty's Government will respond appropriately and robustly."
The Russian Embassy said it was "completely untrue" to suggest the country's special services were involved and criticised Johnson for speaking "in such a manner as if the investigation was already over."
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 be involved in examining what could have caused the 66-year-old man and his daughter to fall ill.
Skripal was convicted in 2006 of passing state secrets to MI6 before being given refuge in the UK as part of a spy swap.