In a statement following an evening of talks in Brussels, the European Council of EU leaders agreed “there is no plausible alternative explanation”.
The EU has now recalled its ambassador to Moscow “for consultations”.
PM Theresa May had said the Salisbury poisoning was “part of a pattern of Russian aggression against Europe”.
Moscow denies responsibility for the attack on former spy Sergei Skripal and his daughter on 4 March.
The European Council statement added: “We stand in unqualified solidarity with the United Kingdom in the face of this grave challenge to our shared security.”
BBC political editor Laura Kuenssberg said the prime minister had “succeeded in getting them to toughen up” their language on the attack from earlier in the week.
But she said it did not seem likely that she will persuade other leaders to take the same kind of action she has against Russia by Friday – although some have not ruled out expelling Russian diplomats.
Mrs May briefed her counterparts on Thursday on the poisoning of the Skripals in Salisbury, who remain in a critical but stable condition in hospital after they were found unconscious on a bench.
Detective Sergeant Nick Bailey, who became ill after responding to the incident, is no longer in a serious condition and has been discharged from hospital.
The prime minister set out the evidence the UK has against Russia – including the positive identification of the chemical used as a type of Novichok nerve agent and the knowledge that Russia has produced this agent within the last 10 years. (Courtesy BBC)