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.
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.
Mr Skripal’s relatives have told the BBC Russian Service that the former spy 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 is based in Moscow and has visited her father in the UK regularly, especially over the past two years.
Mr Skripal – a former Russian agent convicted of spying for Britain – and his daughter were found slumped on a bench outside a shopping centre on Sunday afternoon.
Earlier they were seen walking through an alley leading from a Zizzi restaurant, which has now been “secured” by police. The Bishop’s Mill pub has also been cordoned off as a precaution.
Scientists at Porton Down – the UK’s secret weapons research facility in Wiltshire – are studying the “unknown substance” that is thought to have made the pair ill.
Mr Johnson told MPs: “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 that if it emerged Russia was linked to the incident in Salisbury “it would be very difficult to imagine” that UK representation at this summer’s football World Cup could go ahead in the “normal way”.
Aides to Mr Johnson say he was referring to “officials” – and not the England team.
Mr Johnson said the UK was “in the lead across the world” in trying to counteract a “host of malign activity” by Russia.
But in a statement quoted by the RIA Novosti news agency, the Russian embassy in London said: “Media reporting could give rise to the impression that this is a planned action by the Russian security services, which in no way corresponds to the truth.”