The European Union has backed Britain over the Salisbury nerve agent attack, agreeing that the only plausible explanation is that Russia is responsible.
The decision came after Prime Minister Theresa May gave EU leaders at the European Council summit in Brussels a detailed account of the evidence which led the UK to point the finger of blame at Moscow.
In a late-night statement, the leaders of the EU's 28 member states said they agreed with the UK assessment that "it is highly likely that the Russian Federation is responsible and that there is no plausible alternative explanation."
And they added: "We stand in unqualified solidarity with the United Kingdom in the face of this grave challenge to our shared security."
The announcement came as reports suggested that as many as five EU countries - France, Poland and the three Baltic states - are considering following Britain's lead by expelling suspected Russian spies.
It marked a significant victory for Mrs May, who had faced reluctance from some EU countries with close links to Moscow, such as Greece and Bulgaria, to make a direct accusation against the regime of President Vladimir Putin.
Foreign ministers of the 28-nation bloc issued a statement on Monday voicing "unqualified solidarity" with the UK, but stopping short of pointing the finger of blame at Moscow for the March 4 attack on ex-spy Sergei Skripal and his daughter Yulia.
Addressing the other 27 EU leaders over dinner, Mrs May set out the four pillars of evidence which the UK believe put Russia in the frame:
- The positive identification by experts at Porton Down of the specific chemical used as a type of Novichok nerve agent;
- The knowledge that Russia has produced this agent within the last 10 years and remains capable of doing so;
- Russia's record of conducting state-sponsored assassinations; and
- Britain's assessment that Russia views defectors as suitable targets for assassination.
Mrs May also shared top-grade UK intelligence on the attack with German Chancellor Angela Merkel and France's president Emmanuel Macron on the margins of the summit to discuss the international response to Salisbury.
Lithuania's president Dalia Grybauskaite declared publicly that she was considering expelling Russian spies from her country.
But Greek prime minister Alexis Tsipras was more cautious, saying: "We have to express solidarity to the British people for the Salisbury case, but we also need to be responsible on that issue."
Arriving at the summit, Mrs May said she would tell fellow leaders that she sees the attack on Mr Skripal as part of a pattern of Russian aggression which threatens the continent's democracies and requires a united response.
Wearing a white ribbon in commemoration of terror attacks in Westminster and Brussels, the Prime Minister said: "Russia staged a brazen and reckless attack against the United Kingdom when it attempted the murder of two people in the streets of Salisbury.
"I will be raising this issue with my counterparts today because it is clear that the Russian threat doesn't respect borders and indeed the incident in Salisbury was part of a pattern of Russian aggression against Europe and its near neighbours from the western Balkans to the Middle East."
Meanwhile, in London, Russia's ambassador Alexander Yakovenko condemned Foreign Secretary Boris Johnson's comparison between Mr Putin hosting this summer's World Cup with Hitler's 1936 Olympics.
"Nobody has the right to insult the Russian people, who defeated the Nazis," he said.
Mr Yakovenko demanded evidence for Britain's allegation that Russia was behind the Salisbury attack, saying official statements on the poisoning had been "wild" and the UK had "built its official position on pure assumptions".
The response of the Government had created "serious problems" in the Russia-UK relationship, he said.