May will need to overcome reluctance from the Russia doves in the EU since the bloc is traditionally split on how to deal with Moscow.
Ties between Moscow and the West plummeted over Moscow's 2014 annexation of Crimea from Kiev and support for rebels in east Ukraine, which have triggered sanctions by the EU.
French President Emmanuel Macron said on Wednesday the Salisbury attack could not go without response.
Macron and May, German Chancellor Angela Merkel and U.S. President Donald Trump, have already said in a joint statement they "share" Britain's assessment of Russian responsibility.
Diplomats said EU leaders could to settle for similar language, though some bloc members remained concerned there was not enough direct proof to incriminate Russia in the attack.
European Council President Donald Tusk, who will chair the summit and has sided with Britain, said: "It is clear we should improve our preparedness for future attacks."
He wants the bloc to discuss how to better protect itself from chemical and biological attacks, including in cooperation with NATO, as well as to how to beef up counter-intelligence capabilities to combat hybrid threats.
