European Council President Donald Tusk has siad EU nations will take "more steps" against Moscow over the Salisbury nerve agent attack.
His comments come as the European Union's ambassador to Russia was recalled to consult with Brussels over the incident.
Dutch Prime Minister Mark Rutte said the diplomat was being recalled as a "measure" and the move was not a "sanction" against Moscow.
Prime Minister Theresa May has said the European Council is "standing together" over the Salisbury nerve agent attack.
Earlier, Mrs May won the backing of EU leaders as they accepted the only "plausible explanation" was that Russia was responsible.
Speaking after her address to the other 27 EU leaders over dinner, she said: "I welcome the fact that the EU Council has agreed with the United Kingdom Government's assessment that it is highly likely that Russia is responsible for the attempted murder that took place on the streets of Salisbury and that there is no plausible alternative explanation.
"The threat that Russia poses respects no borders and it is a threat to our values and it is right that here in the EU Council we are standing together to uphold those values."
In something of a victory for Mrs May, the European Council President Donald Tusk said leaders of the other 27 EU member states agree with the UK that it is "highly likely" Russia is responsible for the attack.
The UK is at a diplomatic impasse with Russia, sparked by the nerve agent poisoning of former Russian spy Sergei Skripal and his daughter in Salisbury.
Arriving in Brussels, Prime Minister Theresa May told ITV News: “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."
Britain has been pressing EU allies to follow in its expulsion of 23 diplomats from Russia’s embassy in London.
On the first evening of the EU summit, a Downing Street spokesperson said: "The Prime Minister provided the President and Chancellor with a detailed update on the investigation into the reckless use of a military nerve agent, of a type produced by Russia, on the streets of Salisbury…
"The UK, Germany and France reaffirmed that there is no plausible explanation other than that the Russian state was responsible.
"The leaders agreed on the importance of sending a strong European message in response to Russia's actions and agreed to remain in close contact in coming days."
Lithuanian president Dalia Grybauskaite has announced she is considering expelling Russian diplomats over the attack.
Meanwhile, in London, Russia's ambassador Alexander Yakovenko condemned Foreign Secretary Boris Johnson's comparison of Vladimir Putin hosting this summer's World Cup with Adolf Hitler's 1936 Olympics as propaganda for his regime.
"The British Government is free to make a decision about its participation in the World Cup but nobody has the right to insult the Russian people who defeated Nazism and lost more than 25 million people by comparing our country to Nazi Germany.
"That goes beyond common sense and we don’t think British war veterans including those of the Arctic convoys would share this opinion."
President Putin’s spokesperson Dmitry Peskov also described the comparison as an "utterly disgusting statement which is unworthy of a foreign minister of any country."
Russia is just one of the major topics to be worked out at the EU summit, with the Brexit transition deal looming large.
Mrs May is expected to tell EU leaders it is their duty to show "energy and ambition" in negotiating a post-Brexit trade relationship which will be good for both Britain and Europe.
On Friday leaders of the other 27 EU member states are expected to approve a draft deal on Britain's transition to Brexit, opening the door for talks on trade.
European Council president Donald Tusk raised doubts earlier this week over whether a deal reached by Brexit Secretary David Davis and chief EU negotiator Michel Barnier would be acceptable to all 27 - with Spain reported to be holding out to improve its hand over Gibraltar.
But a senior UK official said on Thursday evening that "all the indications are positive" that the transition deal will get the thumbs-up.