It comes after the Prime Minister, Theresa May, briefed Germany’s Chancellor, Angela Merkel, and France’s President, Emmanuel Macron, on the evidence Britain believes lays the blame for the attack on Russia.
Ms May said the European Council was "standing together'' on the Salisbury attack and "the threat that Russia poses respects no borders and it is a threat to our values''.
Mr Tusk said: "European Council agrees with UK government that highly likely Russia is responsible for Salisbury attack and that there is no other plausible explanation."
The statement agreed by the leaders of the EU's 28 member states at the European Council summit in Brussels said: "The European Council condemns in the strongest possible terms the recent attack in Salisbury, expresses its deepest sympathies to all whose lives have been threatened and lends its support to the ongoing investigation.
"It agrees with the United Kingdom Government's assessment that it is highly likely that the Russian Federation is responsible and that there is no plausible alternative explanation. We stand in unqualified solidarity with the United Kingdom in the face of this grave challenge to our shared security.
"The use of chemical weapons, including the use of any toxic chemicals as weapons under any circumstances, is completely unacceptable, must be systematically and rigorously condemned and constitutes a security threat to us all."
The announcement came as reports suggested as many as five EU countries — France, Poland and the three Baltic states — are considering following Britain's lead by expelling suspected Russian spies.
Meanwhile, the EU's ambassador to Moscow is being recalled for consultations over the Salisbury nerve gas attack, Dutch Prime Minister Mark Rutte said.
Earlier on Thursday, the police officer exposed to the Novichok nerve agent in the attack, Detective Sergeant Nick Bailey, was discharged from hospital.
Det Sgt Bailey said: ”Normal life for me will probably never be the same.”
A court also heard that Mr Skripal and his daughter may have suffered life-long brain damage as a result of the military-grade nerve agent they were exposed to.
The pair remain in intensive care under heavy sedation, Mr Justice Williams told the Court of Protection.
“The precise effect of their exposure on their long term health remains unclear but medical tests indicate that their mental capacity might be compromised to an unknown and so far unascertained degree,” the judge said.
“At present both Mr and Ms Skripal are critical but stable; it is not inconceivable that their condition could rapidly deteriorate.”
A consultant treating the victims as Salisbury District Hospital said Mr Skripal was unable to communicate in any way, but his daughter was unable to communicate in any "meaningful way".