The UK would respond "robustly" to any evidence of Russian involvement in the collapse of former spy Sergei Skripal, Boris Johnson has said.
Mr Skripal, 66, and his daughter Yulia, 33, are critically ill in hospital after being found unconscious in Salisbury, Wiltshire.
The foreign secretary said he was not pointing fingers at this stage, but described Russia as "a malign and disruptive force".
Russia has denied any involvement.
Downing Street said Home Secretary Amber Rudd will chair a meeting of Cobra, the government's emergency committee, on Wednesday to discuss the Salisbury investigation.
Counter Terrorism Police have taken over the investigation from Wiltshire Police.
But in a statement, the unit said the inquiry had not been declared a terrorist incident and there was no risk to the wider public.
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Family deaths
Relatives of Mr Skripal - a former Russian colonel convicted of spying for Britain - have told the BBC Russian Service that he believed the Russian special services might come after him at any time.
His wife, elder brother and his son have died in the past two years, some in mysterious circumstances, the family believe.
Ms Skripal lives in Moscow and has visited her father in the UK regularly, especially over the past two years.
Mr Skripal and his daughter were found slumped and unconscious on a bench outside a shopping centre on Sunday afternoon.
CCTV footage has been released by police which appears to show the pair walking through an alleyway near a Zizzi restaurant shortly before they collapsed.
Police have since sealed off the restaurant and The Bishop's Mill pub was also cordoned off as a precaution. On Tuesday evening, police extended the cordon and shut off a bridge.
Two police officers caught up in the suspected contamination have been treated in hospital for minor symptoms, before they were given the all clear. It is understood their symptoms included itchy eyes and wheezing.
A third member of the emergency services remains in hospital.
Scientists at Porton Down - the UK's secret weapons research facility in Wiltshire - are studying the "unknown substance".
Speaking in the Commons on Tuesday, Mr Johnson said: "Honourable members will note the echoes of the death of Alexander Litvinenko in 2006.
"I say to governments around the world that no attempt to take innocent life on UK soil will go unsanctioned or unpunished."
Mr Johnson said the UK was "in the lead across the world" in trying to counteract a "host of malign activity" by Russia.
Russia has insisted it has "no information" about what could have led to the incident, but says it is open to co-operating with British police if requested.
In a statement,the Russian embassy in London said: "Media reports create an impression of a planned operation by the Russian special services, which is completely untrue."
Responding to Mr Johnson's comments, the embassy added: "Looks like the script of yet another anti-Russian campaign has been already written."