The decision to add the Sergei Skripal case to the summit agenda on March 22 is another display of solidarity with the government of British Prime Minister Theresa May despite divisions over Brexit.
Tusk, who will host the summit as head of the European Council, said on Twitter he had phoned May on Friday “to prepare (a) clear message of the EU on #SalisburyAttack”.
Former double agent Skripal and his daughter Yulia were left critically ill by the attack in the city of Salisbury, southwestern England, on March 4, which May has blamed on Russia.
“Skripal will be discussed Thursday evening next week. At this stage, I expect summit conclusions on it,” a European source told AFP on condition of anonymity.
The incident comes at a particularly tense time for UK-EU relations, as the two sides are locked in Brexit talks ahead of Britain’s departure from the bloc in March 2019.
Western leaders have strongly backed Britain in the wake of the attack and supported its claim that Russia is “culpable”.
Polish Foreign Minister Jacek Czaputowicz on Monday said Warsaw was “in favour of taking action, including sanctions.”
“But that needs to be agreed with a larger circle of EU or NATO allies,” he added alongside his German counterpart Heiko Maas in Warsaw.
The attack has already been raised in recent days at the UN Security Council and in the NATO military alliance, which described it as the “first offensive use of a nerve agent on Alliance territory since NATO’s foundation.”
British Foreign Secretary Boris Johnson is also set to discuss the attack with his EU counterparts and with NATO Secretary General Jens Stoltenberg on Monday.
But while Western rhetoric has been strong, it is not yet clear how far Britain’s allies will go in terms of concrete steps to support London.
One European diplomatic source expected the EU to adopt “similar measures” to London, which has expelled 23 Russian diplomats and cut high level-ties
But the diplomat warned that imposing fresh sanctions against Russia would be more difficult.
The European Union already has sweeping economic sanctions in place against Russia over the conflict in Ukraine and has imposed travel bans and asset freezes on dozens of people.
“More vigorous action would require proof of Russian responsibility,” the source said, adding that it was “more difficult” to attribute blame for a murder to the standard or proof necessary if sanctions were involved.
Download our app and read this and other great stories on the move. Available for Android and iOS.