British police have said Sergei and Yulia Skripal probably came into contact with the nerve agent that has left them in a serious condition in hospital at the front door of their home in Salisbury, England.
"We believe the Skripals first came into contact with the nerve agent from their front door," said Dean Haydon, Britain's' senior national coordinator for counter terrorism policing.
"Specialists have identified the highest concentration of the nerve agent, to-date, as being on the front door of the address," Scotland Yard said in a statement.
The niece of the poisoned former Russian double agent has said her uncle and cousin have only a slim chance of surviving.
Viktoria Skripal said the prognosis for the pair "really isn't good", as the pair remain in a critical condition following the Novichok nerve agent attack on 4 March.
She also revealed that Mr Skripal's mother had not been told of the incident.
She told the BBC: "Out of 99% I have maybe 1% of hope. Whatever it was has given them a very small chance of survival. But they're going to be invalids for the rest of their lives.
"The first priority was to protect our granny so that she wouldn't hear or find out anything."
Her comments came as countries across the world, including Ireland, joined the UK in diplomatic action against Russia, which has been blamed for the attack.
British Prime Minister Theresa May hailed the "unprecedented series of expulsions" of Russian diplomats, which she said sent a strong message to Russia that it could not ignore international law.
The Russian Embassy in the UK said Mrs May had still not presented evidence that the country was responsible for the poisonings, adding that "no-one in the wider world would take British words for granted".
In Moscow, President Vladimir Putin's spokesman, Dmitry Peskov, hinted that the Kremlin would respond with tit-for-tat expulsions, saying Russia would proceed from the "principle of reciprocity".
Russia has already ordered 23 British diplomats to leave in response to the expulsion of a similar number of undeclared Russian intelligence officers from the UK.
Elsewhere, Russia's Ambassador to Australia said the world will enter into a "Cold War situation" should the West continue its bias against Russia.
"The West must understand that the anti-Russian campaign has no future," Russian Ambassador Grigory Logvinov told reporters in Canberra. "If it continues, we will be deeply in a Cold War situation."
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US Defense Secretary Jim Mattis said Russia's alleged poisoning of Mr Skripal is part of a strategy of conducting operations, and then denying responsibility for them, that is designed to divide the West.
He said President Vladimir Putin's denials of responsibility could not be taken seriously.
Govt defends expulsion of Russian diplomat
The Government has defended the decision to expel a Russian diplomat from Ireland.
Tánaiste and Minister for Foreign Affairs Simon Coveney said it was essential that the EU stood in solidarity with the UK and rejected any suggestion that the move breached Ireland's neutrality.
Russian Ambassador to Ireland Yury Filatov said the move was unwarranted and would not go unanswered.
The Minister for Social Protection said the decision to expel the diplomat was an appropriate one.
Speaking on RTÉ's Morning Ireland, Regina Doherty said that the Cabinet had taken its time before making the decision and it was a moderate response to an "outrageous attack on UK soil".
She said the decision was borne out by intelligence shared by the UK, Germany and France.
An Associate Professor in International Relations at DCU said there had been a long standing "interesting" relationship between the Government and the Russian Embassy on Orwell Road.
Speaking on the same programme, Donnacha Ó Beacháin said the Irish decision to expel one diplomat will be overshadowed by larger expulsions by other countries.
He said that expelling diplomats is a predictable response and Russia would have expected it.
Mr Ó Beacháin said the decision may damage the concept of Ireland's neutrality, but that neutrality generally means non-participation in military alliance, and in that narrow definition, Ireland's neutrality has not been undermined.