NATO is set to join more than two dozen countries by expelling Russian representatives in an unprecedented show of support for Britain over the Salisbury nerve agent attack.
General-secretary of NATO Jens Stoltenberg said the alliance would stand united with countries, including the UK and US, by sending home seven Russian staff.
More than 117 agents are currently being expelled in what Theresa May has called the "largest collective expulsion of Russian intelligence officers in history".
On Tuesday, Australia, Ireland and Moldova confirmed they would be taking similar punitive measures against Moscow, bringing the total number of countries to 25.
Russia's diplomatic manpower at NATO has also now been reduced from 30 to 20.
Mr Stoltenberg said the spate of expulsions showed that the Kremlin's behaviour since its annexation of the Crimean Peninsula from Ukraine "has costs".
Australia became one of the latest countries to back the UK by expelling two Russian diplomats.
Prime Minister Malcolm Turnbull issued a statement saying the diplomats identified as "undeclared intelligence officers" would be directed to leave the country within seven days.
He said of the Salisbury poisoning: "It was an attack on the sovereignty of every nation that respects the rule of law and that is why we are taking this action today with another 23 nations around the world, we are defining this recklessness, this lawlessness, from Russia and expressing in solidarity with the United Kingdom and other nations that share those values that we will not tolerate this type of reckless undermining of international law, this reckless assault on the sovereignty of nations."
The co-ordinated move has drawn a furious response from Moscow, which accused Western allies of "blindly following the principle of the Euro-Atlantic unity to the detriment of common sense, the norms of civilised inter-state dialogue and the principles of international law".
Mrs May also told the Commons more than 130 people could have been exposed to the Novichok nerve agent, with more than 50 people assessed in hospital.
Sergei Skripal and his daughter Yulia remain critically ill following the attack, with the PM saying doctors have indicated their condition is unlikely to change in the near future and they "may never recover fully".
Labour leader Jeremy Corbyn said Russia "has a direct or indirect responsibility" for the "outrageous" attack but had offered "denials and diversion" in response to claims and questions over their involvement.
Mr Corbyn was subjected to constant heckling from Tory MPs with former minister Mark Francois bringing proceedings temporarily to a halt when he began whistling the Russian national anthem to mock the Opposition leader.
The stormy exchanges came after Mrs May told the Commons the move to expel diplomats underlined the unity of the West in the face of Russia's deployment of a nerve agent on British soil.
"Together we have sent a message that we will not tolerate Russia's continued attempts to flout international law and undermine our values," she said.
Speaking at the start of a debate on national security and Russia, she said: "Sergei and Yulia Skripal remain critically ill in hospital. Sadly, late last week, doctors indicated that their condition is unlikely to change in the near future, and they may never recover fully."
Mrs May said the UK had information indicating Russia has investigated ways of delivering nerve agents, probably for assassination, and has produced and stockpiled small quantities of Novichok as part of this programme.
In Moscow, President Vladimir Putin's spokesman, Dmitry Peskov, hinted 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.
The co-ordinated move came after EU leaders last week backed Mrs May's assertion that there was "no plausible alternative explanation" other than Russia was responsible for the poisoning of the former double agent and his daughter.
European Council president Donald Tusk said "additional measures" - including further expulsions - could not be excluded "in the coming days and weeks".
The EU member states taking action include Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Italy, Latvia, Lithuania, the Netherlands, Poland, Romania, Spain and Sweden.
Foreign Secretary Boris Johnson said countries of the world "have come together in numbers far greater than Putin could possibly have imagined and they are saying enough is enough".
In addition to the expulsions, the White House said the US was also closing the Russian consulate in Seattle "due to its proximity to one of our submarine bases and Boeing".
Which countries are expelling diplomats?
Czech Republic: 3