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.
- How will experts know if ex-spy was poisoned?
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.
"And 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…
"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.
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He 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."
By Norman Smith, BBC assistant political editor
Although Boris Johnson was careful to stress this was still an ongoing investigation – it's absolutely clear he, and others in government, believe it's highly likely this was a Russian state-sponsored attack.
Pointedly Mr Johnson noted the comparison with the murder of Alexander Litvinenko and said MPs could draw their own conclusions.
The big question now – how does the government respond?
A question made more pressing by accusations that ministers were guilty of appeasement in the wake of Mr Litvinenko's killing.
Mr Johnson suggested there could be targeted sanctions against those close to President Putin.
British politicians and officials could boycott the 2018 World Cup in Russia.
And, perhaps most significantly, in an indication the government could seek to rally international support to make a stand against Russia, Mr Johnson said there could be a co-ordinated response with Nato allies.
Whatever measures are eventually decided upon – if Russian involvement is proven – it's clear relations between London and Moscow have plunged to new depths.
Mr Johnson branded Russia "a malign and disruptive force".
Strong words. But many at Westminster believe strong words are nowhere near enough.
Russia has insisted it has "no information" about what could have led to the incident, but says it is open to co-operating with the police investigation if requested.
Police are currently examining CCTV footage, filmed by a Salisbury gym, showing an unidentified man and woman – thought to be Mr Skripal and his daughter – walking near the location where they were found.
Wiltshire Police has said two 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.
Who is Sergei Skripal?
Col Skripal, a retired Russian military intelligence officer, was jailed for 13 years by Russia in 2006.
He was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK's Secret Intelligence Service, MI6.
In July 2010, he was one of four prisoners released by Moscow in exchange for 10 Russian spies arrested by the FBI as part of a swap. He was later flown to the UK.
Read more about Sergei Skripal's background here.
Putin, power and poison: Russia’s elite FSB spy club
The possibility of an unexplained substance being involved has drawn comparisons with the 2006 poisoning of Alexander Litvinenko.
The Russian dissident and former intelligence officer died in London after drinking tea laced with a radioactive substance.
A public inquiry concluded that his killing had probably been carried out with the approval of the Russian President, Vladimir Putin.
Mr Litvinenko's widow, Marina Litvinenko, told BBC Radio 4's The World Tonight the latest incident felt like "deja vu" – and called for those receiving political asylum to be "completely safe".
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