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.
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