EU leaders have promised an "unprecedented" diplomatic response to the Salisbury nerve agent attack after backing Theresa May's assertion that Russia was to blame.
At the end of the two-day summit in Brussels, European Council president Donald Tusk said the recall of the EU's ambassador to Moscow for "consultations" would be followed by further action by member states.
The reaction from the EU came as the British diplomats thrown out by the Kremlin returned to the UK.
In an indication of a possible escalation in the EU response, French President Emmanuel Macron said France and Germany would be among the countries taking "co-ordinated measures" against Russia, with an announcement due "very shortly".
Taoiseach Leo Varadkar said Ireland would be conducting a "security assessment", with a decision early next week on possible "individual action relating to Russian diplomats in Ireland".
Latvia's foreign minister Edgars Rinkevics joined Lithuania in suggesting his country could follow Britain's lead and "decide on the expulsion of Russian secret service employees working under diplomatic cover".
The moves came after leaders issued a statement supporting the UK's assessment that it was "highly likely" Moscow was responsible for the attack on former double agent Sergei Skripal and his daughter, Yulia, and that there was "no plausible alternative explanation".
It represented a significant hardening of the EU's position after foreign ministers issued a statement earlier this week expressing solidarity with the UK, while stopping short of blaming Russia.
Mrs May, who set out Britain's case against the Kremlin over dinner on Thursday night, welcomed the strong recognition of the threat Russia posed to their collective security.
She said that in the run-up to the summit, Britain had been sharing what information it could through "intelligence channels" as it sought to make the case for Russian responsibility.
"The threat from Russia is one that respects no borders," she said.
"I think it is clear that Russia is challenging the values we share as Europeans, and it is right that we are standing together in defence of those values," she said.
Her comments were echoed by Mr Macron, who told a joint news conference with Germany's Chancellor Angela Merkel: "We consider this an attack on European sovereignty."
He disclosed that France had been asked for technical assistance relating to the Salisbury investigation, which it stood ready to provide.
Mr Tusk said the EU response to the incident was "unprecedented", with the next steps by national governments expected as early as Monday.
"It is very difficult to prepare an adequate reaction to this kind of behaviour with nerve agents. We will never have a real chance to respond adequately because we are completely different to the perpetrators of this attack," he said.
Mr Tusk expressed satisfaction that he had achieved his goal of maintaining a "united" front among the 28 EU member states, despite their different stances towards Russia due to differing geographical positions and political traditions.
"I think it was the best reaction we were able to decide on," he said.
In a hint at the underlying tensions, he acknowledged that not all member states would be taking further measures. Asked how many might take action, he said: "More than one, but I don't think it will be the whole group."
The British diplomats ordered out of Russia in retaliation for Britain's expulsion of 23 suspected spies were now "safe at home", British Foreign Office mandarin Sir Simon McDonald said.
A charter flight flew 48 diplomats and their families - along with four dogs - to RAF Brize Norton.
The boss of the Porton Down defence lab, where the Novichok substance was analysed, dismissed Russian claims that the nerve agent could have originated from there.
Porton Down chief executive Gary Aitkenhead told the BBC: "We have got the highest levels of controls, of security around the work that we do here.
"We would not be allowed to operate if we had lack of control that could result in anything leaving the four walls of our facility here.
"We have got complete confidence that there is nothing that could have come from here out into the wider world."